I've been keeping an assortment of grade 5 and 8 SAE hardware on hand in Plano boxes for years, mostly 8-32, 10-24, 1/4-20, 5/16, 3/8, 1/2. I just keep a general assortment in lengths that I feel like I can feasibly use, sometimes cutting down a longer bolt if necessary. I usually just buy SAE from Tractor Supply by the pound.
Only recently have I started to keep any metric stuff on hand. The last couple of years, I have been tinkering with motorscooters, and recently did a motor swap, and started having to buy a good bit of metric hardware. The problem is that metric hardware is stupidly expensive in the local hardware stores, it isnt hard to wind up spending $50 on what I can fit in one hand, so I have been building up an assortment from McMaster Carr, which still isn't cheap, but a lot less expensive overall, and I know it is going to meet the standards that they advertise(vs. ebay, amazon, etc.). On hand, I keep M4, M5-0.8, M6-1.0, M8-1.25, and M10-1.25, plus some miscellaneous stuff that is smaller, larger, and/or different thread pitch. I also make it a point now to save good used metric hardware from anything that I am disposing of.
I use the waterproof Plano 3700 size boxes, in standard and deep configuration. I mostly use the waterproof ones because I like the latches better.