Ladies and Gentlemen, I'm here to propose a pact.
Always, always, always use Cure I (and not Cure II) in xp grinding parties. Here is why:
In all likelihood there are 2-3 people curing. You are all going to be chomping at the bit to get those cures in first for the sp for them.
Regardless of which rank you cast, your max SP gain is going to be either roughly or exactly the same. You do not have a higher possible SP gain by using Cure II.
Furthermore, your SP gain from healing is based on how much of the spell's potential is used up. So for example, say your party configuration sets your max healing SP at around 100 (5-7 people in appropriate level range). AoE weaponskill hits everyone in the party for 225 damage. You cast Cure I, you heal everyone for 225 HP. For each person for whom you get a SP proc, you get 100 / 100 points.
If instead you use Cure II, you heal everyone for 225 HP and you get 60 / 100 possible points.
Say the AoE hits everyone in the party for 600 damage. By each of three healers casting Cure I, you each heal for roughly 200, each getting 90/100 possible points per proc. Or you could cast Cure II, possibly get 100 / 100 possible points per proc, while making sure that the other healers get 50 / 100 and 0 / 100 per proc.
Using Cure II in that situation is obviously inferior all around, because approximately 1/3rd of the time you will get 0, 1/3rd of the time you will get 50, and 1/3rd of the time you will get 100, for an average of 50 / 100 SP.
If you all use Cure I, you will get 90 / 100, 100% of the time (under the assumed 600 damage aoe example).
Obviously, keep people alive. If the damage is very heavy and you're going to struggle to keep up between 3 of you spamming Cure I, then Cure II is appropriate (e.g., 2 or more monsters attacking simultaneously).
Be kind to your co-healers. Use Cure I. Cure II gains you nothing and screws your neighbor, which also screws you.
Edited, Oct 26th 2010 1:35pm by JayRams