Yes, I would hope whoever checked their flat surfaces would check them from several different orientations.
I don't have a surface plate. I use either my table saw top, my anvil, or the glass top of my desk. It depends where I'm at. For the kind of work that I do any of that is flat enough.
For any garage engineering I am of the mind that you cannot rely too heavily on accuracy. So I try to design with some built in fudge factor. I can try to get things close, but realistically I can only count on getting things so close.
The holes I drilled for these plates are about five thousandths of an inch within each other. For some odd reason one is way out. Maybe there was a chip in the fixture that I didn't catch? That's the way the cookie crumbles.
It still works, because it is all close enough.