An elliptical orifice is always better.......Oh..you meant for the smoker. LOL
Sorry.
It really doesn't make alot of difference, the biggest thing is what works for the size you need vs the space you have to make the hole in.
My rf took the whole space where the cook tank overlapped the firebox. So mine had to be the shape of a tear drop.
I have seen square or rectangle on several big rigs though.