Like others, I've done the -3g to +6g (Pitts) thing wearing prescription glasses. I always wear a "Croakie" (to quote 'Chuffer' Dandridge in an earlier post "thin neoprene headband thingys that surfers wear to keep their Oakleys on"). However, I've never felt that I really needed to, it just makes me feel happier. My glasses haven't shifted at all, even with sustained -ve g, although I did once lose my headset briefly!
(And my lunch on occasion, but that's an entirely different thread....)