We're going over old ground again! I'm not going to re-open the arguments about whether or not the knobs, sliders, wheel or even volume pedal are precise enough.
If you're creating registrations for a song, then use as many as you like. There is no limit whatsoever. If you feel that the tempo jumps are too obvious, then simply use more registrations. My students will think nothing of using eight or more registrations for a change of tempo. You simply daisy-chain all your registration banks together with the Registration Sequence parameters set so that the footswitch changes banks at the end of a given bank's sequence. It takes time and thought, but where's the problem in that? You then know that all your tempo changes are going to be absolutely accurate, every time, and you don't need to take your hand(s) away from the keys.