During my RAF, I used a variety of classes to boost with.

Basically had a lot of toons across two previously existing accounts.
And then added four toons that matched the one I already had.

So had a 70 Hunter boost four Hunters to 60th.
Did the same with a Shaman boosting Shammies.
And a druid boosting Druids... and a Priest boosting Priests.

Didn't do the Warlocks, Mages or Paladins, during RAF.
Although I have Warlocks (4x Warlocks, 1x Priest) in the 30's and Paladins (5x) in the upper 50's since RAF.

Boost a lot of classes during RAF.
More options are great, even if you don't think you'll use them now.
You might want them later.