I use a dedicated tool made and purveyed by Sinclair exclusively for deburring flash holes. A second step I do is uniform the flash hole size -- I find the size variation from lot to lot, and brand to brand of same caliber brass amazing! Lastly, I use a tool called a "crocogater" which, with a few quick twists of the wrist, further uniforms pocket depth. 'Specially with gas guns, that the primer *IS* seated below rim is imperative! Like others posting, I too have not done a "no" versus primer-pocket-cleaned comparison vis accuracy results. But, I do feel better about my loads with the flash hole deburred, a uniform hole size, and pocket size to ensure below-rim seating.
BEST!
georgerkahn