Is it ok to use a shower that just has the backerboard installed and not yet tiled?
Given that you have a pan that is finished and given that your walls have either a membrane or a painted on membrane that is waterproof you could very lightly use the shower.  I would never endorse this if I was doing the job but if you are doing it yourself, then go ahead.  Make the showers short/light and make sure that it is bone dry before tiling.  (This is all given you have a waterproof membrane installed - the answer is no if you will be showering right on the backerboard.)