Most places don't measure they just look. So if you have at least some tread left before the wear marks on the tire it should be okay. If it's close, you need new one's anyway.
Perhaps this explains why I couldn't get a straight answer out of the couple of shops which I questioned over the weekend.

Trying to get an old winter beater safetied & sold, but not interested in spending money that I don't have to. There is ~4mm left (about 1.5mm away from the wear bars) on two of the tires which is enough to keep me happy, but I don't want to play games with subjective inspectors.
Given the MTO's love of paperwork, I expect there's a published spec somewhere, which eliminates any potential debate.