DP & I usually discuss our disagreements in front of the kids unless:
it's about discipline (in which case one of us will give the other the "look" and we'll exit the room to discuss it)
it's about an ex (we have a blended family, so if we are discussing an issue involving one of the dc's bio parent, we don't do that in front of them)
We don't *fight* though - so our disagreements are civil conversations. I think it's healthy for the kids to see adults handle conflict in a loving and respectful way.