If you're particularly worried about a specific post (for instance, you're uncertain of the right word when there are several words with similar pronunciations/spellings, or it's just longer than a normal post), I'd say just ask in that post. And regarding off-topic conversations, I say "meh". Most threads get derailed by far less productive side-discussions. I usually like to tuck corrections into actual conversation anyways. Respond normally, and then as an aside (may be first or at the end) comment on the correct spelling or usage of a word.