Downranking...

...can make sense especially if you're equipped with two pieces of the Tier 5 armor set, which as you can see gives you a return of 100 mana. Spam-casting Rank 1 Greater Heal will cost you around 300 mana if you've got the mana reduction talents and still heal for roughly a big Flash Heal. If your spell fully heals your target you get back 100 mana - a nice little bonus that will help keep raid members up and not burn you out of mana. Consider too that with the new Spirit/Int mana regen, its not unlikely that you'll regen the mana you just spent on that Greater Heal. Downranking might not make too much sense though for a newly minted level 70 Holy Priest - the penalties for downranking spells and the lack of any serious "+ Healing" might make it not worthwhile.

Give it a try and see how you do - playstyles vary of course, but see if you like the addition of a lower rank GH in your spell portfolio.