At the same time. Even now after Shump's 3 month wrist injury that just happened Cleveland is talking about starting Joe Harris. Some guys are career sixth men. Regardless of squad.
Jamaal Crawford for example is a sixth man regardless of his squad because his defense is that atrocious and his offense is so much more deadly (dare I say the real life version of cheesing) against a 2nd unit.
Mostly score first pg's and shooting guards will probably sit the bench wherever they are.
That said, for some guys, you have to set it to 1 season force non-starter, because like James Harden from OKC to Houston shows it's not a 100% rule. A 6th man - non defending 2 guard doesn't always remain that way his entire career..