I was keeping all of mine. I even had the boxes from my childhood (8860, 8841, 8845, etc...). But I live in a small flat, and my "lego room" is right now invaded by a surprize and welcome guest for the last 9 months (my baby daughter..:-) ) and I throw them away (every single one of more than 400) to make room for the new member of our family. Cutting and keeping back sides of the boxes (the ones showing alternative model ideas) would be a very nice idea, which, unfortunately, I didn't realize at that time.
Regards,
Sel