Usually the old seals are OK. They keep water and dirt out OK but are not as effective as say a valve cover gasket. Have some silicone or rubber gasket sealant available so you can fill in any breaks in the seals.
Ghetto the gaskets keep dirt away from the belt of course - one time i had work done on another car and the idiots left a gap in the timing belt covers. I just used the silicone sealer cause there was no gasket made for it.