On 10 we do this with 3 healers. One person for each tank and one for raid/adds tank (phase 1 only).
Phase 1 - Onyxia phase
Onyxia tank healer - be prepared to keep your tank at max health and hotted up. I haven't really found anywhere where you can safely heal the tank and not get electrocuted AND not get tailswiped. So being tailswiped is the lesser of two evils. I stand on Ony's left side while the tank is tanking and when he turns her for her electricity I will get hit with a tail swipe. Fortunately Onyxia doesn't hit that hard so it's not too hard to heal. Just remember to keep yourself topped up too!
Nefarian tank healer - this is usually the heaviest healing of the lot. Run with the tank to the opposite side to where Nefarian is being tanked (usually I throw some hots onto the adds tank while I'm heading there since nothing happens for about 20 seconds) and don't stand where you will be hit by frontal cleaves and breaths! I find forward of the tank and to their right is the best spot (ie don't hide behind the pillar on the left because when Nef is turned you might get the tail swipe there). Damage is heavy so I preheal with rejuv and 3 LBs and then use regrowth every time it procs and I spam nourish on the tank or HT if it's low with swiftmends to help top up. I tend to keep my Wild growths for one of the ranged when electricity goes off, to keep them topped up a little. It's mana intensive so you are going all out until all the adds die and then the offtank healer can come and relieve you a little so you can have some chance to get some mana back. At that point stand with the group of ranged and heal everyone up through electrocute with a wild growth and a swiftmend/efflorescence.
Adds tank healer - follow the adds tank around as they pick up the adds. Now dont' get hit by lightning or tail swiped! I used to think standing in the middle was the best option, but it's not - I think keeping max range and keeping myself towards the nefarion tank and healer saves me from electricity so many times, but of course every now and then I have to run out to heal the adds tank. DPS usually don't take that much damage at this time unless they have an add on them or are standing in the wrong spot (in which case they deserve to die!) so it's not too bad until the adds tank gets to the specified adds tanking spot (so they can all rez there for easier kiting in phase 2) and you just heal them until the adds are dead. It can make your healing easier if the adds get interrupted (warstomps) or frozen with a ring of frost so you can have a second or two to top the tank up but I didn't really need it. Once the adds are down run over to help heal the Nefarian tank as everyone should be over there, and stand with the group and heal the group up during electrocutes. I usually lifebloom the tank and then keep a rejuv up on myself for a quick swiftmend after electrocute. I save my tranq for getting up on the pillar.
Mana tide is good towards the end of phase 1, so if you have it in the raid, use it then.
Each healer will be assigned a pillar so make sure you get to your pillar quicksmart! As a druid it's best if you take the pillar with the most people on it (there usually is one pillar with 4 people on it) as wild growths and swiftmends help heal a lot of people and they can all be hotted up.
Phase 2 - Pillars
It sounds simple enough, get up on the pillar and heal, right? Wrong. Getting up on the pillar itself can be a feat of strength at times! Don't jump in and out of the lava, you will only take more damage. Stand just outside the small ring around the base of the pillar, but not under the spout or you will get stuck on it. I wait till the lava is well over my head, then I head up and then step forward onto the pillar, though most people seem to be able to jump onto the pillar. I tranq as soon as I get up and then wildgrowth and then start laying LBs on someone so I can proc a free regrowth. You can mostly get away with wildgrowth and then alternating with getting an efflorescence down while you wait for the wg cooldown to come back up again, and of course heal whoever is low appropriately. Leave the interrupts for the dps and tanks, unless you are a shaman, though I have no idea how they have time to do that when I am already full on trying to heal my group. It's best to have one electrocute go off in this phase so when they tell you electrocute is coming keep everyone as high as you can. I usually innervate at the start of this phase if I'm not full already, or as soon as I get down to 70% mana because you will need it again later. I usually tree form early in this phase so I can have it up again in Phase 3. Alternatively you can tree form just before electrocute to get off some faster heals. Try to stand away from the others on this tiny platform, every little bit of damage avoidance helps.
Phase 3 - Kiting adds
This can be a real bitch, but it's not so dependent on your healing as much as it is on the tank being good at avoiding. Your adds tank has to be smart and your Ony tank has to be good at turning and having good awareness and communication, as fires on the ground are everywhere.
I use one healer on Nefarian tank, and 1-2 healers on the Adds tank. Once those adds start building up stacks and getting really big, it's useful to cc one just so the tank doesn't keep getting killed, but that shouldn't be for a while. One healer will have to run with the adds tank, the other healer can hang near Nefarian more and help heal the raid because electrocutes are going to go off constantly. So keep everyone topped up as much as you can! It helps when the raid is standing nice and bunched up for easier healing - the stragglers at the edges are what is going to cause a wipe. We bloodlust at 30% and tree form is good in the later part of this stage, when mana is getting tight. Hopefully mana tide will be up again, and if not it's innervate time again. Warstomp and roots helps with cc on the big ones to give you a breather but in general you don't want the adds lagging behind or they get Shadowblazed and they get reenergised up (ie they don't go away!).
Well that's how I heal Nefarian! Having done each of the 3 different jobs on various kills I hope that if you're reading this, this strategy helps a bit with your healing if you are unsure what works best.