Feif Un –
“The MB says if the custom is to daven with a hat, you should. In many communities, the derech is not to wear a hat. So according to the MB, in those places, it’s fine not to, right?”
Actually, I think he says it’s because that is the way one would present oneself before respected people.
source: http://www.dailyhalacha.com/MBerura/Images/383.jpg
If you are not sure (which I am inferring from “right?”) you should ask your Rov/Rabbi.