For ideal group utility maybe something like:

Prot Pally
Resto Shammie
Boomkin
Mage
Warlock

Prot Pally brings so much to the table. AoE threat, group buffs and can rez or heal in a pinch and DI for wipe recovery.
Resto Shammie brings excellent utility with totems, good multi-target heals, good main heals, can rez and self-rez for wipe recovery.
Boomkin brings crit-buffing aura, group buff, a battle rez, is a viable off-tank or off-healer, and has some CC.
Mage brings food and drink, good buff, excellent CC, excellent (multi-school) DPS.
Warlock brings healthstone, excellent mob debuff, excellent DPS and soulstone for wipe recovery plus some CC.

I like the group as you have limited overlap for gear as well. The drood can always pick up DPS leather and the shammie can get an enhancement set if they want to. You only have the mage and lock as DPS clothies. But you still have what is basically a 5 caster group so mechanics are simplified for your gameplay. I suppose you could take a Hunter over the Warlock or Mage but I think that adds a lot of complexity for limited additional utility.

One of the other things that I like about this group is you have the flexibility to respec the Boomkin to Treekin and go Ele with the Shammie. Or you can go Ele with the Shammie, Feral Tank the Drood, and Holy the Pally. Or I suppose go Ret with the Pally and stay Resto Shammie with a Feral Tank (maybe Blizz with buff Retadins? who knows?). Anyway, you get a lot of flexibility in case of rampant nerfing or buffing.

The group I'm leveling has this makeup with the exception of having two mages instead of the mage and warlock. I hate gathering the soul stones and all.

But this in not an easy route to go, certainly. I doubt that I'll be doing all of the heroics, etc., but it should be a nice stable of toons to pull from for raiding and the only limitation on the group is how well they are run.

On the flip side I would say that the 4x Shammie, 1 Prot Pally would be the easiest to run and probably the most efficient. Maybe 3x Shammie, 1 Boomkin, 1 Prot Pally instead. But either way you're good. It really depends on what you're looking to get out of multiboxing.