Right, obviously heat is an enemy of li-ion batteries. My understanding (and I could be wrong) is that if a MBP is plugged in the power bypasses the battery once it reaches 100% charge, correct?
That is my understanding as well. Most of my macbooks (I've owned 3) have been plugged in the majority of the time, with at least some time every few days spent on battery power unplugged. I'd say you're fine as long as you're using the battery at least once every 2-3 weeks -- longer than that might not be good for the battery over long periods of time.
Ultimately if you want the longest battery longevity, keep it between 50% and 85% charged, and keep the machine cool.