Type H for Heals has done some great analysis on how haste affects heal-over-time spells.

One punch line is that, from the perspective of healing per cast time (HPCT) and healing per mana (HPM), haste doesn't improve a spell until you get an extra tick on the spell. For restoration druids, the only relevant extra-tick number is 37.5%. If you get to 37.5% haste -- from all sources including raid buffs -- then your rejuvenation has an extra tick at the end.

You also get a rejuv tick at 12.5% and a regrowth tick at 16.7%, but these are such low numbers that practically all druids will be past those points. There are also several thresholds for lifebloom, but those don't matter because...

The analysis is completely different for tank healing! When tank healing, even if you don't get an extra hot tick, you benefit from haste by being able to cast the hot a little sooner. Thus, for tank healing, you get higher HPS at all levels of haste. HPS will take an extra large bump when you get past 37.5%, but otherwise haste is all good. Assuming you have a 5% haste buff from a raid, you need 1016 haste rating to get there. There seem to be some rounding issues, so it's better to get a little more.

All this said, I'm not sure what the practical impact is. Haste is still great both before and after that number. First, you are probably at least rolling HOTs on the tanks even if you are raid healing, and for rolling HOTs, haste benefits them at all levels of haste. Second, every little bit faster that regrowth gets, the better.