As with nearly all UI questions, the answer is "it depends".
You need to weigh discoverability with user satisfaction, among other things. For example, allowing an invalid action gives you an opportunity to explain why something is invalid. This is particularly useful if the answer to "why is this disabled" isn't obvious. For an application where most users are beginners, that's important.
On the other hand, it can be mightily frustrating to see a control, click on it, only to be rewarded with a "sorry, you can't do that now" message. An app I inherited a couple years back was rife with that sort of stuff and it made using the UI an exercise in frustration.
Completely hiding functionality is probably rarely a good idea. Imagine knowing some feature "was there a minute ago" but now it's gone. Whether it's a menu item or a toolbar button or something else entirely, making it hidden can be an exercise in frustration for the end user.
Try doing a little usability testing, if only by asking the next person you see "hey, does it make sense to disable this or show you an informative dialog". Just one other opinion is often enough to get you to look at the problem from another direction.
Bottom line: do what best serves the user. All the scenarios you mention are valid under certain circumstances. As with all UI questions, ask yourself (or better, your users) what best serves their needs.
