Moderately important. Like, don't worry your butt off about it (I don't even know what a dangling participle is, if that's wrong to use, or anything about it other than that it exists) but run your post through spellcheck (Chrome does it automatically, which is nice) and reread it once or twice if you have to.

It should be clear what you're saying, and excessive typos are not to be had, but minor errors or stuff that's technically incorrect but easy to understand? That's fine.

Also, there's the edit button, for if you forgot to proofread a post.