The worst "cheating" is when people exploit glitches and errors, which usually doesn't affect you. The worst examples I've come across occured before the game came out in the States.

One was a glitch in the AH which allowed people to get infinite amounts of money and the other involved high leveled monsters that had zero accuracy and people would exploit that to fight them at lower levels and gain craploads of exp.

FFXI has a pretty clean environment.