I find that a priority system is the best way to approach healing on a Shaman.. Personally, if i know the fight has a lot of AOE damage i will use the Healing Rain, Gift of the Queen, Chain heal, totems route.. However, when i know the fight is more tank based damage and focused raid damage phases.i stick more with Riptide, HW, and Healing surge. Obvioulsy, i utilize Gift whenever it seems necessary but I dont find it prudent to cast a spell just because its on CD if it costs mana or not. Personally, I find that awareness of an encounter beforehand is the best way to prepare to be a great healer..knowing which phases need AOE Healing and Where the tanks take damage. For example, in M+ i almost exclusively use my CD’s on trash packs since thats where the worst damage occurs.. it is effective to keep your HST totem up on CD regardless of the encounter. In my opinion, Anticipating damage phases prior to them happening is the most effective healing tool..watch your DBM timers for those phases you know need a CD Such as the Orb of Destruction or Slam on Krosus and blow a CD and start casting those big heals right as they are needed..in healing Timing is everything..
Dont’ stress about healing meters…the best raid is the one where all the players are alive and the Bosses are dead..sometimes i find that the ranged are being neglected so i give them the love, sometimes i focus on the Melee and keep my Helaing Rain on them on CD..other times its the tanks that need the love..Be aware! Learn the encounters and be ready! Practice makes perfect!