How long are they? What do you want to make? If you build a rail do you have a hill or do you need a drop in?
You don't really need 2 x 10s but if that's what you got you can def use them. 2 x 4's are cheaper and plenty for most summer setups but if you don't have anything else you need the 2x10's for might as well use em.
Also check out the build a job cult. They have a lot of info on pvc rails and drop ins and backyard setups in general