Normally the vocal or lead instrument is in the center, and the bass also since it is rather non-directional. When I mix, I place the musicians where they might be in an actual performance as you look at the stage.

As far as BIAB, the two most critical tracks to separate are the guitar and piano. However, I may pan them something like left 30, right 20; certainly not all the way right or left. Also, it seems to me that some of the RealTracks are already heavier on one side, so I make use of that in my panning. In other words, if the guitar sounds stronger in the right channel, I would keep it there, because mixing it to the left will actually 'center' it.

BIAB 2024 Win Audiophile. Software: Studio One 6.5 Pro, Swam horns, Acoustica-7, Notion 6; Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus Studio 192, Presonus Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors