As a personal rule, I always back my managers. If I have appointed someone using my best judgement then I know not everyone is going to understand the rational, but I have made it for a reason and sometimes in business you have to trust your instincts.
Some people will not agree with the decisions that your managers make, however I always try and back my team 100% - I know they will have made the decision for a reason and in their opinion they are right. Even if I disagree with their thought process or believe them to be wrong, I will still try and back them; if I question their judgement in front of their team then I risk them losing confidence, which can be difficult to recover from.
If they are wrong then I will sit down with them and have an informal chat, we'll discussion the issue, look at the logistics and try to figure out a better approach for future reference. However, the point is that we work together and I always try to back my people, whether it's the recruitment decision that I have made or their actions.