Regarding that Tom Petty song: I don't see anything wrong with using the spelling "alright" in the title but switching to "all right" in the lyrics. I've done that kind of thing a few times when I transcribe songs to post them in Mudcat.

Whenever you have the choice between two spellings that sound the same, I'd say go with the spelling that your mother would have approved of—or "of which your mother would have approved."