In the rack that I'm using there is one piece of flexwatt. It loops its self back and forth with each level, going across, then down a level before going across again. So in theory, that whole piece should be the same temp and so should all 5 of my tubs.

In the case that there are multiple strips it really shouldn't be much different. If all the pieces of flexwatt are the same size, wired correctly, and the probe is positioned correctly there should be no problems. The probe for a t-stat doesn't go in a tub, it should be attached directly to the flexwatt.