My general rule of thumb is I use registrations 1 thru 4 ( on PSR series) that pretty much corresponds to Variation A B C D. 1=A. B=2 etc. On certain style/era of music I generally use my favorite bread and butter sounds. I’ll sometimes use 5 and 6 for just changing to my favorite voices. No exact science but that seems to work for me and the type gigs I play.