More critical are flattop uppers, as they have a rail designed to accept attachments, and that rail must be fully in-spec for many/most accessories to work.  And the tolerances for these rails are very tight, enough to give a few machineshops a big headache.  Even worse is the fact that the most critical measurements (the 45 degree angles that make up the clamping area) can't be measured without a gauge that can measure from the angles.

My solution is to buy from a supplier who has a reputation for reliably holding to the specs and has good customer service to handle a problem if I do have one.  This policy has prevented me from having a problem in the first place.

-Troy