[asid61]

We use mostly 10-32 on our robot. 1/4"-20 is good for some high-load applications. Dead axle screws.
Nylock nuts are really nice, but get some thin ones too. They come in heights of 9/32" normally and 1/8" (4/32") for the thin ones IIRC for 10-32 screws.

We use a lot of 1/8" rivets. Vex uses 5/32" for the versachassis. 3/16" rivets are interchangeable with 10-32 screws in terms of hole size.