I throw my teams together as early as possible to iron out the wrinkles with macros, movement, etc, and to just get a proper "feel" for how it hangs together.

I even levelled a Gnome Warlock from 1 to 10 in Azuremyst in the complete absence of a Warlock trainer because getting the synergy right is more important than training at the beginning.

Grinding Elites is the best out-of-instance grinding you can do as a group, although there's a dearth of non-instance Elites in the early levels.

The absolute quickest way to knock out those first few levels is to secure the services on a level 70 with AoE (Pally, Mage, whatever), blitz the instances, and swap characters in and out as they level.

I didn't realise HOW spectacular this was until I read a thread about it elsewhere on the forum and gave it a go this weekend with my 70 ice mage. Power-levelling extreme - I was agape at the result.

But for your first team, definitely throw them together as early as possible, even if it's not necessarily going to give you the lowest /played time.

Have fun!