Granted levels expire with RAF, used or not.

I'd level as many combinations to 60th as you can stomach, during the RAF period.
Once RAF is finished, pick one primary team to play.
And maybe an alternate team (different composition so it is quite different) for a change of pace.
Always having rested is a nice bonus too.

If possible, level a PvE viable team as the primary.
You can do heroics down the road, and purchase Bind On Account gear which gets you a 10% experience bonus, which can make later teams much easier to level.
The shoulders (+10% experience) are bought from badges; I'd get the cloth version of these, since all toons can then use them.
You can also get a (+10% experience) chest, from Wintergrasp and they will stack (0.90 x 0.90 = 0.8100) resulting in a 19% experience boost.