My philosophy is that you should never update anything (and I do mean anything) unless you have a good reason to. Simply updating because an update is available is not, IMO, a good reason to update. That's because updates are rarely tested thoroughly enough by the developer that issues them (yes, even Apple), and by updating you run the risk of hobbling an otherwise perfectly well-running system.
Almost without exception, when Apple issues an update (say, for Logic), it will include a note to customers saying "recommended for all Logic users". For me that's just not enough information. It's too nebulous. Going on that scant bit of advice (and it is [i]advice[/i], not a mandate) means that I should trust Apple. Well, I don't. I don't trust any developer. And I say that from experience, not from a position of "bashing".
Case in point: Logic 9.1.4 was recently issued. While it fixed some problems, it broke some other aspects of Logic (notoriously, the LFO's in ES-2). So imagine you were in the middle of a project where you were using ES-2's. If you updated mid-project, say goodbye to the integrity of your production! That's why you must never update in the middle of a project.
Of course, if you made a backup (clone) of your previous system, you could revert back to it. But why go through the trouble? The best approach is, "if it ain't broke, don't fix it".
The two most foolhardy things you can do with a working computer system are to update for the hell of it, and, to neglect to back your system before you DO update.
The example of 9.1.4 and how it was inherently buggy (despite the things it fixed) proves my point that you can't trust an update to be benign. I've seen this situation come up far too many times to inherently trust an update.
So how do you find out if an update is good, bad, or otherwise? First, study the release notes for any update. If it details that the update addresses an issue you've been having, then at least you have some information to go by in making the decision to update or not. However, the best way to find out how well (or not) an update behaves is to research it, and fortunately that's very easy to do! There are tons of people out there who will update at the drop of a hat (unflatteringly referred to as "guinea pigs"). And you can pretty much bank on many such people to post about their experiences with updates on forums (like this one). So it's from their reports that you can learn about the behavior of their systems post-update; from there, you can make a decision. But you have to read many such reports in order to make the best, educated decision. Remember, Logic is a complex program, so one person updating and reporting that "everything's fine" may not be using features that another person uses and which they find to be buggy after updating.