If it is, its probably safer that way.
There's no especial risk in any given system of units; it's mixing systems that causes issues. Which would be riskier - sticking to inches and 'thou' on the Seattle production line, or having a switchover to millimetres - likely on just one programme at that.
"Gee, what's the standard tolerance, 10 (thou) mil?". NASA's inadvertent collision with Mars wasn't because of using metric or imperial - it was using BOTH that caused it. Sticking to what you know is often a good idea.