If you are getting a perscription for glasses then wear the glasses. If you are going for contacts then wear the contacts. If both then wear the contacts and make sure you have them in for some time before going. A good contact fitter likes to see how the contacts look in the eye after they've been there sometime. Mine also likes it to be the end of the time the lenses are valid for , but he's fussy.
If its a contact checkup you may have to take them out, so case solution and glasses is a good plan. By the time they've looked in yoru eyes , had them in/out , put dye in your eye it can be tricky to wear them.