I think it depends on:
a) what you are fighting about. Some topics should not be brought up in front of the kids, including if you are having a disagreement about your parenting methods.
b) how you are fighting. It is not OK to let them see you both hurl abuse at each other.
c) Whether you let them see you make up again too. I think this is important. You need to be a role model and show them that just because you fight, doesn't mean you don't love each other, or won't treat each other with respect. They need to see the both of you being able to say sorry and be 'friends' again.

I don't think it is good for children to constantly be exposed to fighting, but at the same time I don't think it is healthy for them to never see it either; otherwise they don't know how to deal with it when it does happen or when they are in a position where they are fighting with someone else.