Is it important to have common ground between you microcontoller and high-power devices on the same PCB?
I think we'd need to see the exact situation you're thinking about to give you a real answer, but in general,


getting the grounding scheme right can be VERY important, and 
Sometimes getting the grounding scheme right can mean separating your grounds, or even isolating part of your circuit such that one part does not share a common ground with the rest of your circuit.