so its fine to hit drop shots whenever you want, EXCEPT when you think it shouldn't be done. very clear, not arbitrary at all.
My friend, you are sidestepping a couple important caveats regarding this question. No one ever said you shouldn't hit a drop shot. Hit all you want. Hit them as often as you want. The OP asked the question whether it would be considered inappropriate/rude/wrong (impolitic was his adjective of choice) to be leading an opponent 5-1 and choose that time to practice your drop shot.
Look---you are crushing a clearly outmatched opponent. You haven't been hitting drop shots to get to this point in the match. Now, with your foot on his throat, instead of simply closing out the match, you decide to toy with him for a few minutes before finishing him off. That is exactly how the behavior is perceived regardless of whatever justification you try to mount for it. It looks cheap and unsportsmanlike and amounts to little more than rubbing your opponent's face in it. To pick that moment in the match to start practicing your trick shots can hardly be considered anything but ignorance at best and will probably be interpreted by almost everyone as poor sportsmanship.