Say in my spares box I have two screws, a 2.0mm screw and a 5/64 in screw. One metric, one Imperial. Say I lose a Cyclone screw. I screw in the 5/64 into the Cyclone bulkhead. (since the 5/64 and the 2.0mm differ by 0.0something mm it will be impossible to tell apart!) Will I destroy the bulk?
You'll be able to tell the difference just by looking at the screw. 4-40 screws have coarser threads, and 3mm screws are slightly larger. My box is mixed with 3mm and 4-40 and I never worry about mixing them up. (I run both SAE cars and a Cyclone )