Out of curiosity, where would you pan the backing track in that case?
Decent backing tracks will be stereo with the instruments already positioned across from left to right in the way that the producer wanted or thought was natural, so you don't want to mess with that. Usually the solo instrument or voice you add to the backing track would sit in the middle, but that is often where the drums and bass already are.
If you are making your own backing tracks, for instance with Band in a Box, you may get the flexibility to position each backing instrument where you like. BiaB Styles come with the rhythm section instruments already panned to a good starting point. But you can move them around. And you can also take account of where the rhythm section instruments are positioned when choosing the best panned position for your solo instrument. For instance if drums and bass are central and piano is panned 25 left, you might place the sax 25 right.
When I am recording my four saxophones for a quartet without rhythm section backing track I currently use:
- Alto 70% left
- Bari 20% left
- Soprano 20% right
- Tenor 70% right
Hope that makes sense