Piston ring gap installed measure

[1034am]

Hello, sorry if this was covered, but I couldn't find any posts on it.
I am installing piston rings snd wanted  to know if it's really necessary to check the installed end gap on new rings. I checked uninstalled end gap & it was good (measured to large end of allowable specs.)
Pistons and cylinders are used, approx 13,000 mi. I am replacing valve seat and figured since I had everything apart I would do the rings also (new from BikeBandit.)
Anyone have any experience or opinions on this?
I have feeler guages & a micrometer but not sure how to measure on installed piston.
Thx!!!