Would also say roll what you want tbh. Sure, the guilds currently raiding are going to have most of what they need. That said, attrition is a thing and every guild we slowly need to replace people over time. Also, there are new guilds forming all the time. Jump on early and level/gear with them. That will most likely guarantee you a spot when they start raiding.
Final note and prot warriors. They are in high demand for 5mans, so you will be able to find runs whenever. Valentine makes a good point about gold requirements for BoEs, but don't let that scare you. Any guild worth their salt is going to make sure you are geared if they want you to tank for them.