Having powered through 41-51 in dungeons yesterday, I find AST extremely fun. Here are my rules for cards.

Ewer = RR (I am finding less and less times I need the mana)
Balance/Arrow = Spread (I spread on boss fights or at the end of trash fights if I don't have a good RR buff up. It's worth a 30s wait if I can get a spear or arrow for aoe buff or increased duration from the others.)
Bole = Use on tank or RR or spread but I prefer to save spread for Balance/Arrow
Spear = RR for AoE RR (Things may change at 60 but haven't needed my CD's that much)
Spire = RR or unless it is an AoE pull with a Bard or DRG then I will give it to them.

I find less times to be able to dps as I have mainly had DRK tanks and they seem to require more healing since most don't know how to manage their MP to keep darkside up. Our Regen is also not as strong as a WHM and thus provides less ticks during larger pulls. I have use Nocturnal but it felt lacking in comparison to Diurnal. Either that or I just prefer the Regen.