At the beginning you will really need a dedicated healer for heroics. Once you beef your tank alot you will not heed healing at all or almost no healing, so the healer will be slacking doing nothing.

I have a pala tank and 4 shamans. All the shamans are dual spec elemental and then resto.

Here are things I can do:

1 tank + 4 DPS --> all heroics and some naxx bosses I can solo.
1 tank + 3 DPS + 1 healer --> OS 10 no drakes, ICC trash becomes a joke with this.
1 tank + 4 healers --> Lets say that I had a fight with Patchwerk that took 45 minutes.

Basically I switch from elemental to healer shaman when I see that healing to the tank makes a dent in the overall dps. If I have to heal the tank too much, then the group DPS drops alot. Switching to a dedicated healer allows for easy mode dps feast, but it's only good if your DPS is hurt enough to justify a dedicated healer.
Usually in almost all instances I don't need to heal my tank at all. For example, I can make a full gundrak run without tossing a single heal on the tank, not even on boss fights. 3 x healing totems + seal of light + judgement of light usually is more than enough to keep the tank topped all the time.