I don't have post editing abilities (and probably won't for a while) but I fancy myself to be fairly capable in the English language and I want to know for when I start editing: is it OK to perform massive surgery on a question/answer if I think I can understand what the person is saying?
Totally. Go to town - no one's gonna shed a tear over a hard-to-read question gone to make way for one that gets the point across.
The way i look at it, there's a revision history and a one-click rollback feature - even if you totally screw up, it's not hard to correct. The benefits far outweigh any risk.
I believe most users will find such activity acceptable as long as you improve the clarity of the post. However fixing minor errors like
you're might be seen as abuse. If people are answering the question without any fuss, it's probably alright. If it's mine-field of spelling and grammar errors (not just the occasional instance), then fix it up a bit.
It should go without saying that if a post is completely incomprehensible after you read it through, fix it up -- but only you know what the user is implying, if on the other hand you have no idea what they're after, leave it for someone else who might.
It's fine (even encouraged) so long as the edits are justified.
The problem can be that people will start editing and changing what is essentially a style difference. This is at least in part understandable. After all, as programmers we tend to prefer the way we code over what someone else does. But you have to get over that.
If it's a good edit, the OP will generally appreciate it (or at least not be put out over it). If you're making stylistic changes, it's akin to barging your way into a stranger's house and rearranging his furniture. I've seen plenty of examples on SO of this sort of thing putting people out and it might go a ways to explaining why edit/rollback wars just seem to follow certain people around.