What about just making 1 or 2 heal buttons and press them whenever you are supposed to heal? Spamming heals (even if there is a pause) is just wrong because it does not only waste mana, it also generates unnecessary threat.

With my priest I only put healing-over-time on my tank when he's getting a lot of damage. Normally he just gets a greater heal when necessary.
I have 1 heal button for (*):
- normal heal when pressed normally
- fast heal when shift is pressed
and another button that (**):
- does healing-over-time when pressed normally
- that does AoE healing (instant) when shift is pressed
- does AoE healing with cast time when alt is pressed

2 buttons and all you need :)

Normally I only use the first button and in heavier fights I use the second one too. In specific situations I might use the alt/shift modifiers to get specific heals.


(*) Something like:
/cast [nomod] Greater Heal
/cast [mod:shift] Quick Heal

(**)
/cast [nomod] Healing Over Time
/cast [mod:shift] Instant AoE Heal
/cast [mod:alt] Slow AoE Heal