I agree that there's a line, but when there's something obvious like missing <code> tags, or a closing markup tag, it seems like a waste of time and effort to consider the node and wait until enough people make a "well, duh. fix it." vote. I don't wait. I fix stuff like this when I see it.
I also, on occassion, change node titles. Usually, this is to fix obvious typos. On occassion, it's to make the node title either appropriate ("help me" isn't), or findable via search. I usually leave a note at the bottom of the post, and usually /msg the author. So far, nobody has objected, and i've gotten several "thanks" from people who hadn't thought about searchability when titling their post.
If there's a concensus among editors that this is over the line, I'll stop.