In my experience it just sort of settles down into whatever works.

Even with Mary as a duo, we've found it's best for me to be on the left as seen by the audience (is that "stage left"? I forget) as apart from anything else it means when I'm playing cello and shes playing banjo (a common combination) (a) I can hear the banjo better because it's facing me and (b) the long end of the banjo isn't in my way.

I'm sure practical considertaions like this always determine the final outcome.

With our ceilidh band, the line up across the stage from L to R as seen from the dance floor is:
Gina (recorder)
Dave (fiddle)
(mixer is usually behind Dave or between him and me)
Me (melodeon)
Mary (keyboard)

The factors that probably led to this are:
- We are two couples so we like to be next to our own partners
(some would say that's a bad thing...)
- Band is led from fiddle so he's best in middle
- keyboard takes up space so better to have it at side facing slightly inwards

...all of which information is no use to anyone else, because every band is different in instruments, who does what and personalities.

Anahata