I've never had a problem wearing my glasses on coasters. Usually, I put a strap on, and I tighten it when I get up to the station. With that on, there's absolutely no way they'll fall off. I did, however, ride Millennium Force twice with my glasses on, and no strap, and they stayed on. My second ride (in the front car), I had to reach up to push them on further (just precautionary), but they never flew up and away. Just buy an adjustable glasses strap that can be tightened, and away you go! (I'm much more worried about losing my contacts on a ride...the wind can just whip them out.)
Jman