There are times when fudging the roll behind the screen is a good thing to do. Especially early on when the PCs are relatively weak and squishy. Also, sometimes you know that the crit you just rolled would put an end to the character that your player WANTS to keep playing to see how the build pans out.
The other thing I will point out, though, is that if somebody ever complains/questions, pull the screen and force them to take whatever rolls you make right before their eyes.
It's one or the other so they can't complain if they force you to take down the screen.