First off very nice design, they look very well built and turned out great.

I HIGHLY recommend a thermostat setup. Dimmers(rheostats) are inferior to thermostats no matter how you set it up. With this rheostat setup you will have to be constantly monitoring and adjusting the temperature to keep it stable. If the ambient temperature of the room increases and no one is there to turn down the rheostat what was once a appropriate temperature can climb dangerously high.

If each bin has a identical flexwatt setup there shouldn't be much, if any difference in the heat output to each bin.