My powermatic (vertical) has a hard polyurethane tire on the bottom wheel,
and is supposed to have a crowned upper tire. However the upper tire is
worn concave. Still tracks OK, I just have to keep the upper wheel tilted
back enough that the back edge of the blade rolls on the rear guide wheels.
Considering I routinely push with 50+ lbs force on the blade edge, it's not
like dragging on the blade rolls is a problem.

As for tracking "free", without benefit of the blade guides, one crowned
wheel should be sufficient, it may just be a more finicky adjustment than
two crowned.

Bob