I find that collets provide a good quality control device for me.
Frequently I will machine a section of a shaft and then part off and
flip the part around and machine the other end. This requires that I
hold the machined part in the collet that matches its machined
diameter. these are typically standard sized collets, but gripping on
a machined rather than stock diameter. If the correct sized collet (ie
the size of the spec'd shaft) does not hold the work precisely, the
piece is not to tolerance and typically is scrapped. These are
relatively simple parts but with many people of different skill levels
performing the machining, if the collets don't hold I assume the part
is worthless.

Woodworker88
Student Machinist
Los Altos High School "Eagle Strike" Robotics Team
www.lahsrobotics.org