Just to clarify here, for both raids and dungeon bosses, pre pull Regen is 100% 'optimal' from the perspective of trying to maximise your DPS. If the tank is pulling with their face rather than a shield lob or such then that's on them. But for a white mage, being able to spend your entire PoM duration right off the bat throwing nothing but stones is a very significant gain. Not to mention, with a pre pull regen, dots are applied and I'm throwing rocks from the second GCD of the encounter, if I wait until the tank has tapped the boss I'm not throwing any stones until the 4th GCD at best.
For trash, again, it pains me to say it, but from my very extensive testing, a mid pull regen is 'optimal' in terms of DPS potential. However, that's absolutely reliant on your tank being able to cope with the agro. Some can, some can't and this is absolutely critical to work around.
I'll openly admit I'm very cautious about early regens at least for the first pull or two whilst I get a feel for what the tank is doing, however I'll absolutely be dotting things up whilst we are moving and I'd expect any decent DPS to be doing the same, the agro from that is a little more delayed giving the tank a chance to tag stuff first.