What sort of investment is good to make in Concentration and Balance? Is there a point at which, for either skill, you're better off just not investing at all, if you can't make a breakpoint (like for AC)? Is there a point at which, particularly for Balance, you just don't need more?

I'm working on a 16 brd / 2 brb / 2 rog build, and, trying to work in trapmonkey skills, skill points are naturally a limiting factor. So I'm trying to figure out how much I can get away with shaving points off Concentration and Balance.

The design intent includes combat healing (Empower Healing, etc), so I don't feel I can just dump Concentration and declare that I don't need it because I don't cast in combat.