I don't think that the 501st or FISD are against screen accuracy, just that - especially in the case of ANH TK's - they were built so haphazardly (we keep forgetting that Star Wars was a low-budget '70s film) that if they were to allow every variation that showed up on screen it would devolve into 'anything goes'. There are troopers with 10 teeth, troopers with 6, troopers missing tube stripes, TKs with unpainted ab buttons, stunt troopers with four rivets on their belts (instead of two), missing holsters, missing thermal detonators, troopers with shoulder covers put on backwards, some held together with nothing but duct tape... so instead the focus is on the majority depiction in the film.
TL;DR: Yes, there was a trooper with 10 teeth. Doesn't mean it has to be approvable. My two cents.