Is it considered rude or bad form to either comment on or edit an original poster's question title in order to fix a grammatical error?
The entire point of allowing anyone to edit posts is so that mistakes can be fixed. Because your edits will need to be approved by other users, make sure you fix as many problems with the post as you can. Try to leave as good an edit summary as possible, since edit summaries are public (the original poster is likely to see it, so don't be rude!).
Those are the very basics, so if there's anything else you want to know feel free to ask me!
For extremely in depth information see: How do suggested edits work?