A box adds $50-$100 when reselling, so I do keep them all. I buy used when I can and often or not, there's no box which is a shame.
I've rotated through numerous lenses, buying used then selling them again if I didn't like it or didn't use it as much.
Over the years, I've only held on to several lenses that I both use often and won't sell.
70-200 F/2.8E
24mm F/3.5 PC-E (will get rid of it if there's ever a newer better version)
95mm and 105mm Printing-Nikkor/Rayfact lenses
Schneider Xenon-Sapphire 97mm
I might even sell my Mit 5x and 10x. The 20x is definitely going, found a better copy.
Out of all these lenses, I only have the box for the Mit 10x which I purchased new and the 70-200E which was also bought new. I do have a 17-35 F2.8-F4 Tamron, impressed with the quality, but I'm sure I'll sell it one day as well. I have the box for it too.