Regen is a superb spell, I aim to keep it up constantly when I know it will get its full duration used before the end of combat. It ticks seven time with a 150 potency, for 1,050 total potency, and only costs 186 mp. Compare this to cure II, which has a 650 potency and costs 266 mp, and you can start to see how very efficient it is. Since it's a HoT, it also pulls much less hate than a cure bomb at the beginning of a pull, allowing the tank to secure hate easily.

As for divine seal, use it on CD for every single trash pull, there's no reason not to. When used with regen, it will affect the potency for the entire duration of the HoT, regardless of whether or not divine seal wears off before regen. This is adding about 300 potency to your regen effect, roughly the same as a cure, for free, every pull. When things start to get really messy, divine seal> medica II has obscene healing potential across an entire raid. Just be sure to keep shroud on hand, because stuff will hate on you if you aren't careful.