I always considered rule one of healing do NOT pre regen before a pull. It's not worth disrupting your tank's pull for one tic of regen. On WHM I start my (snail slow) DOT cast and usually by the time it's about 75% through cast time all mobs have red names thus pulled as needed, then my cast goes off. I'll regen after that.

I don't use Medica II much unless everyone else is also taking damage or there happen to be AOEs among the pack I think may hit someone other than the tank. Always remember your tank is fighting you and the DPS for hate on top of keeping mobs clustered so try not to overheal and make things harder than they need to be. We all have an emergency instant heal if it comes down to it.