I've noticed some closed questions get edited to fix formatting or spelling, without changing the content in any substantial way. This puts them in the reopen queue, where they will inevitably get closed again.
I also get the urge to texify or typeset a question deserving of such treatment so I get the feeling (and I've been guilty of this in the past). But since that edit won't make the question acceptable, I think they should be left in peace.
If you don't think your edit will get the question reopened, there's probably little use to editing it.
However, if you think someone else could build on your edit to reopen the question, definately edit.
The goal of closing questions is to give the asker time to rephrase. Sometimes there's no real hope of that happening, either because the asker is too muddled in their thinking or posting some sort of rant. (It's often hard to tell the difference.) But more often, askers just need a helping hand to lift their question into an answerable state. If the formatting of a question makes the intention unclear, it can sometimes help to fix the formatting so that someone else can fix the deeper issues. (And I've even found that "someone else" to be myself.)
I don't tend to make small edits to closed questions, but I'm pretty sure I've done it before (haven't checked).
I think there certainly can be a time for it: for example some questions closed as "too broad" or "unclear what you're saying" can be drastically improved by clearing them up, and then if the O.P. clarifies the issue will be good-to-go.
Other times I've looked at closed questions that I personally don't think should have been closed in the first place. In those cases I think I'd put the effort in to clear them up, since in my mind they shouldn't have closed in the first place. Related to this is that I've seen questions in the 'reopen' queue that I've voted to reopen not because they're good questions, but because the reason is just completely wrong (eg a very clear [yet off-topic] question that might have been closed as "unclear what you're asking")
Ideally, closed questions should not stay closed forever. Instead, one of two things should happen:
the question is edited into shape and reopened, or
the question is clearly off-topic or otherwise unsalvageable, and is eventually deleted.
The main exception to this rule are questions closed as duplicates, which generally should be kept as "signposts" to direct visitors to the "canonical" version of the question. Duplicates are different from other closed questions in many other ways, too, and should really be considered as a separate case.
In fact, the Stack Exchange engine automatically deletes closed questions that have not been upvoted and don't have upvoted or accepted answers. (This also happens to non-closed questions that have been lying around for a very long time with no answers, upvotes or signs of activity.)
Despite this, in practice, most Stack Exchange sites do seem to accumulate a considerable backlog of closed-but-not-deleted questions. This is mostly harmless, as long as those questions stay out of the way, but they do sometimes clutter up search results or otherwise get in the way. When you spot such questions, you can help keep the site clean by voting or flagging them for deletion (or, of course, by editing them into shape, if you think they can be made into a good and on-topic question).
Anyway, the upshot of all this is that, IMO, the only good reasons to edit a closed question that I can think of are:
because you think you can fix it so it can be reopened, or
because it was closed as a duplicate, but needs some fixing (e.g. retagging) to serve as a good signpost.
In most other cases, if you come across a poor-quality closed question, you should simply vote to delete it (or flag it for deletion, if you lack the rep to cast delete votes).
As a particular special case, if a really poor question happens to have a good and on-topic answer, my suggestion would be to edit the question to match the answer (rewriting it from scratch, if necessary!), which should hopefully make it eligible for reopening. Alternatively, if the question is a duplicate (even if it wasn't closed as such), you can flag it for moderator attention and ask for the answers to be merged to another, better question.
