I'm fairly competitive so as a rule I don't tend to hold back. Sometimes I will against friends so that they won't decide they never want to play with me again, but that usually makes things less fun for me.
Against random people online though the usual sequence of events is I dominate them for a bit, get kind of bored and then I start doing crazy off the wall things that would never really fly against better players. This probably makes me seem like a bit more of a dick to people who aren't that good though because then I'll still be crushing them, possibly even more so because nobody expects off the wall stuff that shouldn't work to actually happen, let alone end up working.
I never do it for the sake of being a dick though. I'm just the sort who gets bored if I'm not consistently challenged and will make up my own challenges when other players can't provide them. Strangely enough, making up my own challenges has often made me a vastly better player than most people, like when I stopped zooming in as the Sniper in TFC, or when I refused to use anything but the crossbow in Turok 2, developed the ability to headshot anyone within a fraction of a second of them rounding a corner, and then realized that the crossbow is actually the best weapon in the game.