It's still better to use fire every round, because otherwise the fast healing half of regeneration will make the encounter run longer. Even if it only adds one round to the fight, that's one more chance to get hit with a rend.
Even in 3.5, once you got a troll unconscious at -10, you could still just coup de grace them with a torch or acid splash, and they'd be just as dead.
I like that PF makes it a lot easier to handle if the troll (or party) manages to escape and come back later, you don't have to keep track of the prior fight's damage as the troll just regenerates it all and is back at full.
And the troll would need to be below -21 in PF, not -10 (the troll's con score). And if the troll had endurance and diehard, he'd never fall unconscious (although he would be staggered), so the party might never know when to turn off his regen with acid or fire.
Even in 3.5, once you got a troll unconscious at -10, you could still just coup de grace them with a torch or acid splash, and they'd be just as dead.
Not completely accurate, the acid or fire damage would have been real damage, all the other damage done would have been subdual and would come back. Even the coup de grace only worked if it failed the save as the subdual damage done would have eventually gone away leaving a couple of points of lethal damage from fire/acid from the last attack.
On the other hand with the new rules, when regeneration is up, the creature cannot die. Doesn't matter what you throw at it, no death spells or effects, massive damage rules, whatever. It cannot die.