The good thing about Regen is that it stacks. Medica II and Regen can be stacked on top of another healer (WHM) so it should be all right. Your usage of Medica II -> Medica -> Assize is the best way to top everyone off.

Overheal is something that you should care about, but only if your weaknesses are clearly seen.
I will tell you some Overheal examples that are glaring enough for you to care about:

For example, you spam Medica II too much. Even if it works, your MP would be drained too quickly!

Another example, you spam Cure II on a target whose HP is full or almost full non-stop. Your MP will drain too quickly too! Because you healed too much and gain little (Overheal), you should try to be more efficient.