Supplements as a general need: I rarely tapped into mine unless my notes/outline had gaps or I just didn't understand something.
Editions: depends on how dramatically the law has changed in recent years. There was a recent revision of the federal rules of civil procedure, so you probably want to get the newest edition for that. I think for most things you could probably get away with being a year or two behind, but I wouldn't go much farther than that. If you're ever unsure, shepardize the case in question with your school Lexis/westlaw account.
Your professor will likely tell you the law changed recently, so your cue will be what's in your casebook and anything that the professor points out. Just be mindful of that.