For Disease/Poison/Curse/Magic dispelling in my Raid group, we usually have a priest take care of the Magic while I cover the rest as my Resto Shammy. Some people swear by Decursive, but I tend to like Grid+Clique setup to show only the ones that I can remove with a 5-button mouse.

In general, the person giving out healing assignments should also know some details about the various healer classes, and set up the groups to best benefit the group make-up.

Specifically, I find myself having to remind the raid leaders that certain totems (the pulsing ones) don't affect the entire raid, and only work on the people in the same group as the Shaman.

I typically ask the leader to move my Shaman around based on the fight we are on. For Grand Widow Fraelina and Maexxna in Naxx (and most of the spider-wing), I have them put me in the same group as the melee/tank so that the poison cleansing totem can clear them when I can't. This little change in group can make a big difference, especially when Maexxna is doing her soft-enrage while poisoning the tank and webbing up the raid. This has saved me from having to waste a GCD on removing the poison from the tank before using my insta-crit-big-heal macro.

For most other fights I have the leaders put my Shaman in the same group as the healers, excluding the Healadin in my guild that seems to have a static blue bar beneath his health, that bastard...