I replace the metric nuts and bolts right off. I do not want to take a chance of getting them mixed in with my other hardware. I keep a rather large supply of hardware as I tend to build at night, so repacement is not dificult.

If I have to make a field repair I know I will have the right hardware in my field kit.

I do put the metric nuts and bolts away where it can't get mixed with the other hardware and save it for emergencies.

I tend to upgrade hardware on most of my planes, metric or not as many come with cheap parts.