I haven't run into anyone who cheated, but I have run into a couple of bad sports. Childish stuff, really; like throwing your ships and templates around when you have some bad luck, or sulking excessively when you lose a match. Plenty of people have a love-hate (often mostly hate) relationship with their dice, but this guy was having a bit of a tantrum when his dice failed him.
That said, the vast majority of people I've played against take everything in stride and can have fun even in the face of comically bad dice. I played a game last week on Vassal where a cloaked phantom was taken out at R3 by two Y-wings, having rolled 5 eyeballs followed by 5 blanks, and we both had a good laugh about it. Presumably he burned his computer ritually later that evening to appease the dice gods.