I've never had a problem, but I don't stray much beyond +5 and -1.5g. In an open cockpit aeroplane I would strongly recommend using a cloth or leather helmet, if not a bone dome - not only to these help hold your specs on but they also keep your headset attached. Some pople use microlight kit, others use the various soft & hard helmets compatible with DC headsets and others.
Do make extra sure you can reach your spare set though.
TKF - It's up to you, but if decide to spend a lot of time in open cockpits I'd suggest glasses instead of contacts, or perhaps a very good set of goggles over your contacts. It can be very dehydrating with a lot of breeze swirling around.