For TSO PG. I cant speak for other brands.
what Packard said about blade thickness and, that white strip on the rail, if it moves or is worn from using different blades from when you first calibrated the PGs that will throw your measurement off a tad.
also Using different rails will throw the accuracy off the white strip has been cut differently all are not the same so check prior to using
So as a force of habit. I always check to be sure the measurements and the calibration of PGs are correct prior to cutting.
The same way I would check my fence on my table saw for accuracy prior to cutting.