It all depends on how big of a load you have on the tires and the shape of the tread. If you need 80psi on a 32mm tire you might get away with 65psi with 38mm but the first thing I'd suggest is getting a long bungie to secure the panniers to the rack so they can't bounce around. The next thing is to get a tire with a wide tread and knobs on the sides like the Marathon Cross or Continental Travel Contact. so it can't skitter around riding on a center tread as is common for street tires.