You have three options for gemming for HPS: Haste, crit, or spell power. Haste will always give you the greatest return on itemization for increasing your HPS between the three points with the current gear available.

Writing a guide for healing a 5 man or heroic is not an ideal way to go. The marginal difference and impact of how you gem or gear for this content is not significant because gear obtained from most heroics will trivialize the content itself. Thus, trying to optimize a strategy that applies only to that content constitutes wasted resource and only disseminates misinformation about how to do things when they get to raid content.

With that in mind, gearing and gemming for more haste is beneficial regardless of the quality of your raid. No, this does not mean that you gem +20 haste in every slot and forgo every other potential benefit. At this time though, haste will provide you, point-for-point, a higher return on itemization points as long as you are actively healing. Mana is largely a non-issue in most encounters regardless of the quality of your raid as long as you have the necessary raid buffs to support healing.

If your typical raid has the following: replenishment (two sources for 25 man), improved blessing of wisdom (or your own totem), 5% critical strike (boomkin or elemental shaman), spell power buff increase (demo lock or elemental shaman), 3% haste (swift retribution or improved moonkin form), and wrath of air totem (5% haste), then there is little to no potential for you to go out of mana as long as you’re managing your cool downs appropriately. With this as the case, you should always be safe to trend to gem for +haste over other HPS increasing mechanics (Spell power or crit).