Personally, I think it's perfectly fine to disagree in front of children - *about some things*. Points to consider:

- Disagreements occur all the time. Parents can model healthy ways to disagree and to solve conflict to their children. 
- Your child will disagree with you all the time. Pretending parents don't disagree might send a message that adults get over that kind of thing, which would create an unrealistic world view for the child. 
- Disagreeing respectfully helps children learn that they won't lose a parent's love because they have different opinions. Learning to think for oneself is a life skill that's inhibited if parents pretend disagreements don't occur between two people who love each other.
- No one is perfect. Imagine if parents had to support each other in what is an obvious mistake. If there is disagreement, there's an opportunity to model forgiveness (as well as apologizing/asking for forgiveness) in front of the children as well.

I would say, though, that parents should provide a united front at all times in the matter of establishing rules and consequences for children, and discuss differences of opinion in private. This helps to prevent the child playing one parent off against the other. 

**Edited to add** in response to a comment:

We used to have family conferences - including the kids - where we all contributed to the discussion and made joint decisions together - on what was and wasn't allowed, why (if this was a top-down decision) and appropriate consequences. The kids were often given new privileges this way, and when they broke the rules, they knew they had already agreed to their appropriateness before, so it cut down on the "no fair!!!" aspect. This to me is more about decision making within a family (which does involve some disagreement), but not what I would categorize as parents disagreeing in front of the kids.