I am thinking of taking the "only keep boxes for products I still have" approach.
That's the approach that I take. I keep the boxes for stuff that I'm likely to sell later on down the road to fund something else, or the ones that are small. I did make one exception to the rule though...
Here's my main small pile of boxes in the closet, along with some random Apple bits and bobs. From the top down is my 2012 mini, AppleMouse IIc, 2011 Air, and 2012 rMBP with iPhone 3GS, 4S, and 5 boxes. I don't have boxes for my Mac Pro or G5, because those are huge and I don't plan on selling them. The rest of that area is mostly PowerBooks that I'm working on restoring, and an old G4 mini.
And here's the exception. It's large, but it's a pristine, in-box iMac G5 with all of the accessories