We've hidden all metric tools in a box. A few mentors and trusted students know where it is... most team members don't know about it. If a student really needs a metric wrench, they'll ask a mentor and we will give them the wrench. We haven't stripped a bolt head using a metric wrench in years.
As for the topic of which bolts we use, 1/4-20 was our standard frame fastener, but we're moving towards rivets and smaller bolts. We've had issues with people not distinguishing between 10-24 and 10-32 (and 8-32). Better organization, training, standardization, and one of
these should help us.