I could never understand those who felt the need to cheat while playing an rpg, example "Congratulations, you fudged your funny shaped plastic die roll so your imaginary half elf just avoided an imaginary decapitation from an imaginary orc" (?!). Doesn't this kinda reek of pathos?
We typically give a 1d4 roll to each player at the beginning of each session, which the GM watches and records.
We term these 'luck' rolls, and a person can use up one of these to replace a bad roll, or spend two 'luck' rolls to force the GM to re-roll. I think people have less of a tendency to cheat knowing they have this safety net, and the GM doesn't have to worry about looking over someone's shoulder every time someone picks up a 20-sided.
But every group is different, and every group member has an idea of what is acceptable and what is not. To that end, I suggest that it is a good idea for everyone -players and GM alike - to set agreed-upon parameters of what will be tolerated and what won't in a gaming session, before beginning.
At the end of the day however, its just a game and should be more about having fun than anything else. Players and GM's alike should feel comfortable in their group, not suspicious and paranoid or feel like they have to cheat to get ahead.