As long as it's not constant, and as long as everyone's having fun, it's not a problem.
I've seen a few cheaters in my time, some more blatent than others. My favorite had to be one SW D6 guy who threw way too many little dice so only he could read them, and would mysteriously claim to be using a force point if anyone asked. It was a problem only because it prevented other people from doing things because he managed to actualy do it all. I've seen others do it as well, and while it didn't initialy detract from the fun, it wore on everyone eventualy (Particularly the DM, who was trying to chalenge the PC, and failed because he fudged a series of rolls, at least it appeared he did.)
Have I fudged a die role occasionaly? Sure. But I try to be honest, and there have been times I've fudged a roll the other way (Fudge a 19, then roll a 19, I'll fudge it down.) I do it more often from the DM side, because I hate having lucky rolls kill players, but I've done it as a player too. I'm sure other players I play with will on occasion too. But I'm the kind of DM who will tell a player to reroll if the dice are killing them that night (I've seen someone fail their best save roll 7 times in one night, it wasn't pretty).
That's part of why I like Action Points and things like that. Why cheat, when there is a legal way to fix my bad roll in a critical situation. When the Dice take the fun out of the game, what's the point?
Worried about honestly? Ask people to use normal or larger sized dice that are marked in such a way that they can be read at a distance of farther than 5 inches. Personaly, uber-mini dice or unmarked dice realy bug me, because it seems like they're used explicetly for cheating.