The answer, IMHO, is it depends. I look at it like this, if the instrument, in your case the fiddle, has a mic or an amp I leave it where it is. But if it doesn't have a mic or amp then I move it towards where the singer was standing.

So if you are trying to emulate a true blue grass band then I would move the fiddle to a little left of the singer when it takes the lead: note this is from the audience's perspective. I do this as I have seen the singer stand directly in front of the mic but when the fiddle/instrument's lead comes in the singer will move slightly to the right and the fiddle/instrument will be slightly to the left of that mic. That is very slightly to the left. YMMV

I arrived early to a restaurant.
The manager asks, "do you mind waiting a bit?"
I said no.
"Good", he said. "Take these drinks to table 3"

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware