I keep everything, usually doing a final Saveout of all my sessions for Backup which has everything neat and tidy (because sometimes there's a lot of scattered saveout seession files for Reel deliveries and such.
Each show has it's own master folder on my drive so I archive that (thus including OMF, spotting notes, sessions, etc). I'll usually do a quick cleanup of un-needed bits like temp prints of design samples for approval etc. Pix is on a third drive for me, following the same master folder scheme, so I archive latest pix too (and older versions if necessary).
If I was just contracted to do a part of a show, like sound design or sound effects editorial (and thus I don't have extra materials), I'll do the final Saveout of the edit session (making sure any and all Stage Addz/Replaces are living in that session, so that saveout is a sort of "final snapshot" of what was shipped to the stage). Usually I'll always leave the guide track living in this saveout, since more often than not I'm having to make volume tweaks and little edits here and there so the guide's not distracting as I work through a show. That way if you have to load up the show again down the road, you can simply open it and you're up and running.
I recommend to always archive, and keep the material on your active work drive until the printmastering is finished - god only knows how many times we receive a call at the 11th hour for some sort of Addz either due to late VFX or a clients request, so if you have to switch over to that show at a moments notice, best to have it nearby and not packed up with a pretty bow just yet.
Archiving is definitely a must, or rather, an investment in future job security - if not for running into a situation where the material is needed years down the road, but also as a good practice of organizational and data management skills.
And sometimes, it's nice to know where maybe you built something unique a few shows ago and know exactly where to snag the source edits from archive to save time. I've found that helpful, especially for a cigarette/candle 'smoke trail' design I made a while back. Constantly grabbing that saveout of the source edits, huge time saver.