Really depends on the guild you are in. In my raiding guild in Vanilla we had a ton of mages. So many that I couldn't guarantee each of them a spot in the raid. I was the Mage Class leader...so I always had a spot if I wanted it. We also had many Warlocks and Shaman...and not all of them got to go all the time either. Not many Rogues or Warriors...so the ones we had were guaranteed raid spots. Other guilds will have different compositions though...you might find yourself in a guild that is heavy on Warriors/Rogues and light on Mages/Shaman. There is no real way of knowing off the bat which class will get you a raid spot.
Some people will say there is always room for more of "X" class...problem with that is that if everyone rolls "X" class than there will be a shortage of other classes...and all of a sudden those classes will be the ones in demand.