Necessary? No.

I'd start using your preferred addon now. I use healbot and I started using it on my priest as early as level 20. It just makes things a lot easier and as you level up and start healing Wrath dungeons, you'll start needed the utility of switching quickly between multiple targets who all need your attention to abolish poison, decurse, heal, and maybe even res.

I would say though, that they are necessary if you're doing some of the harder (ie, new) 80 dungeons or heroics.

No addon is necessary. But most make certain activities more streamlined and that will apply to raids and 5mans as well.
Grid2 + Clique is what I use and like. VuhDo is very popular with healers in my guild too. Most people I knew who used Healbot switched to either Grid+Clique or VuhDo.

I'm new to tree healing myself (and a lowbie @65). I couldn't say for sure if it's necessary for 5mans or not, but certainly it appears it *will* be necessary for raids, so why not get started now while there's some room for error? Fiddle with a couple mods (I'm using VuhDo) and see what you like. By the time you start doing the harder content you'll be a pro (or at least I hope *I* will by the time I do, lol)

I've used Grid+Clique since level 60ish on my druid (after being yelled at by my boyfriend), and now I find it so useful I'm even using it on my lvl 15 priest. There is an amazing guide for customizing Grid for druids on Resto4Life.com - Grid can be a major pain in the ass but Phae's explanations are very good and her setup works perfectly for me.

Awesome. Well, I guess that confirms it!
I started thinking about the fact that I should just practice with it now after I posted. ;)

I'll try configuring Grid and Clique and start practicing. Thanks!

One last question (I don't want to hijack the thread), my balance gear will translate decently to resto with the +spell power, correct? Or am I going to be geared wrong going in and trying to heal these things?

Never mind, Found this article over on Restokin.
http://www.restokin.com/2009/12/what-is-the-difference-between-moonkin-resto-loot/

Using a healing addon will make healing more fun, because the addons make it easier to heal and also see everything that's going on around you. I assume you're a druid (since this is the druid column) -- for a druid a healing addon is very nice for tracking the duration of your HoTs. If you plan to eventually raid-heal, learning and setting up Grid+Clique is worth it. If you aren't, Healbot will work well out of the box and is good for 5-mans. I healed my way through instances while leveling (as feral) and I still remember the first instance I healed with Healbot. The difference was amazing.

And for your followup--yes your balance gear should work pretty well to heal in while leveling.

Mouseover macros are a must, regardless.

#showtooltip
/cast [target=mouseover,exists][target=player] Regrowth

I'd recommend you try Lifebloomer, and set up mouseover macros for your healing and decurse/abolish poison spells.

It might take a little adjusting to recognise a curse or poison (or both) effect, and the /lb party command that only needs to be run once one you're in a part might be initially annoying.

But I love how it shows quite clearly how many stacks of LB are on, and showing how much time remains on the HoTs, and an indication of how much damage a player is taking, even if they're being maintained at full health by your HoTs.

I stick with healbot for my pally heals, but Lifebloomer seems to be king for me.