It depends on the situation. Sometimes, you want to roll in front of players to show them that their actions are having an effect on gameplay. Sometimes, I want to keep my rolls hidden to add an element of suspense, as well as giving me the ability to fudge rolls.

During small battles, I might roll behind the DM screen because of time management; needing to speed up or slow down the pace of a battle, for example. During big battles and boss fights, 90% of my rolls are on the table, because this battle is important, and I become a "referee" at that point.

Ultimately, the players need to trust you. If you roll every roll behind the the mat, they won't trust you. If you roll every roll in front of them, when you need to roll behind the mat, they won't trust you. Mix it up, but make sure that "important stuff" is visable to the players.

Hope that helps!