Perfectly sensible. I tend to use one registration bank per song, but for genres such as quickstep, ballroom waltz where I may do a medley or not depending on audience response I do have mu!ti-song registrations.

So as ever, do whatever suits you best.

The only other thing I can add is that I'm finding the SX900 touch screen makes registry editing and saving a joy compared to my Tyros 3. I'm happy to allow Korg PA owners a smug grin at this point!