I guess everything depends on your preferences. If you want to smash through instances by pressing as few buttons as possible 5 x shamans is probably the way to go. If you want a more complex and challenging experience a mixed group is the way to go.

I always play mixed groups. I have about ten characters that I try to keep at about the same level (they are between level 60 and 65 now). I always use the same protection paladin tank. I have two healers to choose from; a holy paladin and a holy priest. And then I throw in any 3 DPS's from my pool of fire mage, destruction warlock, elemental shaman, feral druid, fury warrior and combat rogue. Whatever I feel like to play that day.

There's a lot of things to keep track of when playing five different classes, but so far all instances up to Underbog have been doable. I don't know about heroics though, perhaps they will be too hard for my mixed groups.