Its the perimeter that counts, but round is the most efficient...I'd do it, but assure that 36 inches overall is sufficient....so length matters, as well as the size of the rooms, insulation, and comfort levels..
Too small may result in noise...
A poly material may have less friction.
This becomes complicated, does it not ?
We are only considering a 5 foot run, so worry not.
I did learn something at school years ago...