Two particular settings that I often see recommended are "Write hierarchical keywords" and "Write catalog data to IDimager ICS scheme". I don't remember ever finding or reading arguments against them, so I will assume that enabling both of them is good practice. (We don't need to take this as gospel: if someone disagrees, then please explain why.) This said, I would like to raise here the more general DAM issue of writing parent keywords to the standard keyword field (xmp:dc:subject) and the PSU specific issue of parent label assignment. (They are not quite the same thing, although they are closely related.) I often see recommendations against those practices (generally dismissed as "not needed"), but I am not convinced their pros and cons have been considered and compared in detail (in a single dedicated thread, that is). I am laying down here my own analysis - hopefully it's not too flawed or hard to follow.
Part 1: Writing parent keywords to the standard keyword field (xmp:dc.subject)
Pros:
(1) Allows external tools and platforms that do not support hierarchical keywords (e.g., Picasa, Flickr) to read and expose all the applicable keywords (for image grouping, searching and filtering purposes).
(2) Increases the number of keywords per image and therefore the chances to find the cataloged images on commercial sites, platforms etc. (This is probably a pro for stock photographers. LATER EDIT: The previous statement is in question - see Mike Buckley's comments bellow.)
Cons:
(1) Duplicates information already available in the hierarchical keywords field (lr:hierarchicalSubject).
(2) May lead to an overly large number of keywords. Less is more (...unless you're a stock photographer?).
(3) May lead to keywords with a weak relevance (or even completely irrelevant) outside the PSU cataloging.
(4) May have a small performance impact in PSU (although it's likely negligeable).
My comments:
As you could see, most of the cons (1-3) actually refer to the same drawback: informational noise. A big case is sometimes made of the duplication with hierarchical keyword field. However, I do not fully understand the argument, because the complete keyword hierarchy info is available in the lr:hierarchicalKeywords and, as long as it is used, any flat keyword in xmp:dc.subject (including a leaf keyword) is by definition redundant (not needed). Similarly, storing the ICS schema introduces duplication and redundancy too (w.r.t. keywords), but it is considered good practice (and I do agree with this). I am therefore not entirely sold on the redundancy + duplication cons arguments, as long as I regard the first pro (that is: non-reliance on only those platforms which read and interpret correctly the hierarchical keyword field) to be a quite important one. Am I alone here?
Part 2: The automatic assignment of parent labels
Assuming a PSU user does want to write the parent keywords to the image - presumably because he/she has determined the pro(s) to outweigh the cons above - (s)he has at least three ways to achieve that using the PSU labels:
1) Assigning the parent labels manually
2) Enabling the global setting: "Include all parent level labels as keywords" (can be overriden only via private labels)
3) Enabling the setting "Also assign its parents" in every leaf or midlevel label (with possible exceptions)
The first one is obviously quite error-prone (assuming the parent keyword writing is desired across all - or most - hierarchies), while the second one is pretty much transparent in PSU. I would like to discuss the pros and cons of the third, which I think is the most controversial.
Automatically assigning parent labels
Pros:
(1) Allows easy filtering by parent label for any collection (using the label filter or the new text filter).
(2) Allows easy searching by parent label (using the search bar), optionally in conjunction with other criteria.
(3) May lead to useful label suggestions inside LAP. (N.B.: this is just my hypothesis; could anyone comment on that?)
(4) It may be used in conjunction with other label settings (such as field mappings or applied profiles) to implement a powerful workflow. (N.B.: this is a pro only for advanced users who know what they are doing; it could easily turn into a cons.)
Cons:
(1) Increases the number of assigned labels, making them harder to manage in a number of ways. (Two specific examples bellow.)
(2) Increases the number of labels in LAP, and that could make it harder to spot which labels ought to be added and which labels ought to be removed (for a given image selection).
(3) Increases the list of labels available in the label filter dropdown, possibly to the point where the list becomes overloaded and requires scrolling.
(4) Conflates the information provided by hierarchical and non-hierarchical label views inside Catalog Explorer (i.e., clicking on the image counter vs. to the left of it). Sometimes it can be useful to discriminate between the content provided by those two view modes (e.g., see the next point).
(5) It is (obviously) incompatible with a (arguably useful) convention that requires only leaf labels to be assigned to images.
(6) The current handling of automatic parent assignment is very fragile with respect to structural catalog changes (label merges + relocations).
(7) The setting may (unintentionally) interact with other label settings (such as metadata mappings or applied profiles) and lead to subtle, unpredictable and/or undesired side effects.
(8) May have a small performance impact.
My comments:
The list of cons against the automatic assignment of parent labels is quite long, but I find the first two pros to be quite significant. It is great to be able to quickly filter on a label and know you get all the images under that hierarchy. (Yep, the hierarchical search is available also in the catalog explorer + dynamic searches + favorites - it's just that the filter bar is very convenient and I use it a lot.)
Conclusions
Well, despite undertaking this analysis (mainly for my own decisions and benefits), the bad news is that I haven't actually reached a firm conclusion yet. It seems to me that the case for or against writing parent keywords - and how to implement this decision, once determined - is not clear cut and hence it is still fair game for debate. (Actually, a fair conclusion might turn out to be: it is a matter of user's priorities. But, if one clearly favors simplicity, he should better not worry about parent keywords and labels.) As far as I'm concerned, I'm not sure if I should proceed forward using automatic parent asignment or not. (The bulk label editing in V3 has made the auto-assignment option tempting and apparently easy to change and experiment with, but that facility does not actually simplify the implications for label changes and for already cataloged images - see cons 6 above.)
Am I overthinking this? (I bet I am.) Any comments, corrections or conclusions are more than welcome. (Please keep in mind, though, that a worthy goal for most users is to end up with a shorter overview, not a longer one.

Have a great weekend,
Vlad