[Lil' Lavery]

It depends. If the fan applies force directly through the center of mass of the robot (vertically speaking), then no torque would be applied to the robot and no traction characteristics would impact the wheels. If the force is applied beneath the center or gravity, it would create a torque that would apply additional force to the rear wheels and remove force from the front. If applied above the CG, the opposite would occur.

Ultimately, it's not different that acceleration of your robot via a standard drive (which, since the force is applied beneath your CG creates a torque favoring your rear wheels). The additional force on one set of wheels should be identical to the force removed from the other set, so the normal force of the robot as a whole won't change (just of individual wheels).