Since I am a player and a DM, there is a group of spells that I know what they do but cannot remember if it is a Will or Fortitude save, and since I do not ever play an evil character they are not spells I use regularly, only when I DM.
I would also like to add that even if you do not know the spell that the save is against, you can still have the PC's roll to determine if they saved, because quite often a save equals no effect.
Believe me, I have to look spells up all the time for their different effects, but if there is no real cost difference, and it helps speed up the game a little then I think it would be a good thing.