This is a question I am regularly asked. The answer really depends on many factors. Clearly the first being if you can see without them. If you are completely blind without your glasses, then clearly you have to wear them or consider contacts.
Girls with big noses may want to keep them on since it can break the line of the nose. Girls I never realised had big noses, took off their glasses on their wedding day and well there it was.
I always try to take a few without glasses since glasses reflect light and the flash. This can make it hard to see the eyes, which are generally the focal point of a portrait.