Downrank that Heal

I mostly use Greater Heal Rank 1 as my "main-heal". And throw Greater Heal Rank 7 only if max heal is needed. With Renew+Prayer of Mending and occasional emergency Flash Heal, I'm set.

There's no point to overheal if downranked spell does the job.

This saves mana, and thus, might be the edge you need to conquer some battles.

Especially as a Priest, you will still need to dance in-out of the 5 Second Rule (Mana Regen at WoWWiki), but don't overlook downranking.

What I do is this:

I have macroes for max and min rank for each spell (and some middle ranks too, like GH Rank 6).
I cast different ranks of spells on myself several times, and count the "average heal" for each rank.
I then update the macro names like:
"2000 GH1" - for Greater Heal Rank 1 which heals 2000hp on average
"3800 GH7" - for Greater Heal Rank 7 which heals 3800hp on average
.
.
etc.

I update the macroes/buttons every now and then when gear has upgraded.

So when I place the buttons on my action bar, I can easily see what I need to cast if I need to heal, let's say, 2500hp or 1200hp = I can throw big or small heal depending on what is needed.

I use Clique for heals, but the buttons are there to remind me that Greater Heal Rank 6 heals this much and Flash Heal Rank 1 heals this much and so on.

In most cases you do fine by spamming the max rank, it saves mana and makes you more efficient healer.

For theorycrafters, it's about Heal / Mana. Use the Priest Healing Calculator to see the differences of the ranks.