I am on the OCD list.
I measure left, center, and right sides of the quilt.
Take an average of the three measurements.
Cut the first borders several inches longer than the length of the center of the top
Fold the border strip in half - mark the center point - then measure to the ends to the 'exact' length of the center
Then I divide/fold the 'needed' part of the border into fourths and then eighths
I also divide the edge of the top into eigths and match up the segments and pin them at those intervals
I usually end up really close - but occasionally I'll get a bit of creeping on a long stretch - and I'd rather cut off a couple of inches on an end than be 1/4 inch short
Then I measure the the other direction which will include the attached borders.
Bottom line: I cut the border strips a bit longer, but mark them 'exactly' before attaching them.
This is for the butted - not mitered - way of doing borders.