Personally, I prefer going into prot for Divine Guardian... Having Beacon and Sacred Shield on the same CD is helpful and saves you the GCD of putting SS up every 30 seconds.
Divinity is underrated in my opinion and sits at the very top of the prot tree. As a tank healer, strong and consistent heals are better than a few extra crits throughout the fight, so 5% healing there beats the crit from the ret tree.
Also, if you DO NOT have a prot paladin or resto druid in your group (yes, this can happen in 10-mans) you should try to have Improved Devotion Aura from prot as well for an additional 6% healing.
Finally, at lower gear levels (and in all of 3.2) you'll be spamming flash of light rather than holy light, so if you can get in some arena for the libram that gives +267 spellpower to FoL, do it.
I recommend "Clique" which allows you to bind your heals very easily... Ex: shift+Lclick is FoL, shift+Rclick is HL, shift+mouse5 is shock, alt+Lclick is Beacon and alt+Rclick is SS
Combine this with an amazing raid frame like X-Perl and you'll be set. Seriously, with this setup, I sometime top the chart on effective healing
Personally I found prot spec most useful only after I got t8 4 pc bonus.