If players can't see the DM's dice, there may be reason to believe the DM is fudging.
They can believe what they want. But they'll never know the truth or be able to metagame the DM based on his rolls, or even metagame what or why he is rolling in the first place.
A player that is concerned about the DMs rolls is taking an adversarial position with the DM, and to me, that's indicative of a deeper problem at the table.
IME, truly great DM's use the dice as a tool. Bad ones let the dice dictate the reality.