That depends on what kind of account you're building, what your stats are currently at, and if you plan on making it members in the future or not. For F2P, though, it's always a good idea to get pray up. Prayer can be the difference between a kill or a death. However, prayer training on F2P is a big money waste, considering you have to bury your bones and you can't use gilded altars. If you have the gp to spare, definitely do it.