Do:
Regen when all mobs of the pull are on the tank.
I always wait till the Tank aoes and the mob names change colors because i'm that lazy and i don't do regen to early to make stuff run off
If you happen to have regen on the tank when adds spawn, like they do in Kugane Castle, just plant yourself near the tank or in the MIDDLE of where the adds spawn. Less work for the tank.
Same goes for bosses, i wait till the tanks start running towards them and are about to attack, then i cast regen on them, like the exact same time as their attack hits, at leat i'll try
Divine bension/Tetra/Bene whenever all the mobs of the current pull cycle are on the tank and the tank has too much damage in between for some reason.

Don't:
Regen while the tank is still running around. Expecting every tank to aggro EVERYTHING while they're still on the way to their destination doesn't always happen and it doesn't need to.
Regen at the end of the pull. Nobody expects you to be a master in managing every regen every time because it happens that some groups just clear trash waaay faster than others.
I did get pre pull regen punched out of my system because i got told that thats where i get all the hate at the beginning from(noob me didn't know back then, the times)

If you tank yourself its pretty annoying if somebody insists of having regens on you all the time. I click it off if i have it at a bad time but a lot of others don't because always thinking of doing it isn't easy.
And most healer run away from the tank instead of literally bodyslamming them to help them not to have a konga line of mobs behind them.