You only need to change the belt if yours is worn, or if you just want a fresh one, my oppinion is they don't cost that much, and you if your car has many miles on it you might as well do it while its off. One note be VERY careful with the crank bolt and carefully follow the directions, there are many people that have messed up their crank online from not reading the directions when dealing with the crank bolt.
Ike
p.s. you will need a torque wrench for installing your crank bolt (some people say you can get away without it, but remember if you mess up your crank the whole engine has to be disasembled to get to it)