Calipers: Short answer - Yes!
I would say the vast majority of people who have been turning pens for more than a year, and All that sell in high end markets use calipers for determining the size, fit and finish. Commercially available bushings are too unreliable to use as a gauge for fine fit. - exception is the custom made ones.
One of the many advantages of Turning Between Centers (TBC) is that it eliminates the need for finishing to the size of a bushing - which diminishes in size every time a piece of sandpaper touches it while turning, or the chisel touches it while turning to size. It has long been known by the pros that bushings are "consumables" on the mandrel for these very reasons.
AS to TBC, finish sizing is done without bushings and only with calipers, and then the finish is also applied without the bushings, sooo . . . no sticky bushings.
Other problems eliminated by not using the bushings after getting to near size are "Off Center" bushings, bent mandrels, tail stock too tight, and other problems eliminated. Less is better!
Hey Micah, I was in Searcy this past weekend. My daughter, SIL and 5 grandkids live very near there in Augusta. I go there about every other month at least!