Once they know both, the general rule is to use side Bb if the note right before or after is a B or C. Any time you have a mix of B and Bb, use side Bb. If you can avoid flipping from 1st to 2nd finger by using side Bb, do so.
That said, a lot of professional sax players use bis Bb almost exclusively. We have to flip all the time from B to C anyway (yes, there's side C, but that's not always practical) so we're pretty quick at it. If it's a really fast passage, I follow the rules above, but in most band lit I just use bis.
Things you should not do - these are badly out of tune:
1 and 1 Bb
bis Bb + side C key for a C