I've been running 4 x locks and 1 x priest for a little while now and hit 22 with them last night. I started out just grinding mobs and doign the occasional kill quest and then hit RFC until 18-19 or so. Since then I've actually started questing. I've been following James Leveling guide (Horde) and just skipping the collection quests. This seems to be the best strategy as it maximises XP and since I've got a full group all of the quests are very easy to complete even if they con 1 or 2 levels higher than me.