I wouldn't overclock it unless you really want/need those few extra frames per second that make a noticable difference because yes, it makes it run hotter (and shortens the lifespan as a result)
Personally, I'm actually for underclocking if you don't play that many 3d-intensive games, to reduce overall heat and power comsumption, and bump it up manually to normal speed whenever you do play and again if it would make a visible difference. I'm not sure if there's software to do this for you automatically depending on what you run yet