Libram choices can be sticky, especially since swapping in combat now triggers the gcd. Since we can't do that it's usually better to choose what you think will be best for a particular fight and equip it first. Every once in a while I will still swap mid-fight if I can spare the cooldown. I also recommend that you play around with the various librams and blessing combos in a non-combat situation, just to look at the numbers. Generally, however, here is what I do:
General use is Blessed Book of Nagrand. Definitely has been worth the horrible grind of Nesingwary's quests.
If I know that ALL my targets are going to have Blessing of Light, then I use Souls Redeemed. You get a nice little boost on all of your heals that way. As I said, check it out and see for yourself.
If I think we're in for a longer fight where I'll have to use a higher percentage of Holy Lights, I may equip Libram of Mending (20 heroic badges, I believe), which gives you 30 seconds of 22mp5. Before the change to Libram swapping I had this macroed to my HL key, BBNagrand to my FoL -- it was great. Now I use this Libram much less.
As for your actual question (which I just realized I didn't answer) -- it really depends on how you're doing. I ran Botanica (regular) with a protection pally last night. We found that she wasn't taking heavy damage, and didn't need Light -- the group got more benefit from her having two 'offensive' buffs. Afterwards we went to Heroic Slave Pens. In this case we found that she needed the extra healing that Light provided, as her gear is really on the bottom edge of heroics at this point.