Having leveled 6 different teams up from 0-70 with RAF, here's my take:

From levels 1-60 if you have a 70 available, use it. (Best PLers: Hunter from 1-20 for Aspect of the Pack speed and one-shot kills, Prot pally from 20-45 in SM then 45-60 in Strat) This should cut your leveling time by 2/3 (you can get 3 teams of 4 toons PLed to 60 in the time it would take to get one team of 5 to 60 in my experience - I've done it both ways).

From levels 60+, a 70 doesn't help much. Might speed things up by 5-10% - definitely not worth losing another level 70 toon at the end.