As you can see in my answer here, I am in general a minimalist when it comes to editing others' posts. However, when it comes editing questions to improve grammar, spelling and punctuation -- and especially when these mistakes seem to stem from the OP not having a native level proficiency with the language in which they are writing -- I think it is much more valuable to make the edits.
- It makes the post easier for others to read and understand.
- It elevates the level of discourse on the site, which contributes to a learned and professional atmosphere. [Anecdote: yesterday, while waiting to get blood drawn, I couldn't stop staring at a sign that contained the phrase "furthur precautions". I finally realized that it was actually freaking me out a little: it eroded my confidence in the competence of the lab. And despite the fact that everything went smoothly, in the back of my mind there is still the idea of going somewhere else where the spelling is always comfortingly correct!]
- It may in fact teach the OP something. In my experience on math.SE, most regular users who are not writing in their native language are well aware of the pitfalls of this and thankful to have others looking out for them. [This may be the most site-specific part of my answer. The mathematical community is truly worldwide, but there are not yet many high quality non-Anglophone math Q&A websites.]
- You are losing the distinctive character and/or humor of imperfectly written language.
- You are messing with what are not mistakes but in fact intentional stylistic choices on the part of the OP.
As for #1, I don't personally view this as much of a disadvantage at all, since as above, in my experience most serious users who write imperfectly wish they wrote better. In particular, they probably don't want to be laughed at or taken less seriously because of their writing. I do take #2 very seriously though.
How do you know when your edits are actually cramping someone's style? Ask. If there is any doubt in your mind, you can leave a comment describing your edit and asking if the user is okay with it. Conversely, if someone edits your answer to fix a "typo" that was actually intentional on your part, you can leave a comment explaining that what you wrote was not a mistake. As usual, maintaining a polite and collegial tone in such matters goes a long way to keeping everyone happy.