Every assembly line that puts a washer on a screw by hand has an opinion about the hole.
Make the hole too tight and the operator fights it. The washer rides up on the thread and seats crooked. Make it too loose and the washer slides off the screw before it reaches the part, and at the end of the shift somebody is picking washers out of a tray under the conveyor.
For metal washers the standard tables give a clearance that suits most hand assembly. Soft washers give you another option. A fibre or rubber washer can be cut with a hole slightly smaller than the thread, so it grips the screw and stays on. The operator picks up screw and washer together, as one piece, and the dropped washer problem goes away.
We cut a lot of these for electrical fittings. The inner diameter ends up being the dimension everyone argues about, and it is the one that decides whether the line runs smoothly.
If your washers keep falling off, or take two hands to fit, send us the thread size and what the washer is made of. The fix is usually a few tenths of a millimetre.