I always assumed every good healer would know to time there regen in a way so that it just runs out exactly when a pull is finished or at most 2-3 seconds later. Yes, you waste an extremely marginal amount of MP if you recast regen too early, but if that's all you have to pay to time the end of regen perfectly with the death of the mob, it's a small price to pay.

I just find it kind of sad that most tanks don't even know they can click it off. When I play as dps, it's annoying when healers do something like this and the tanks don't know how to deal with it and either just stand around silently for 20 seconds or just pull anyway and then I have to try and chase the monsters around.

I've already seen really rude tanks too, though. They click off regen by default. Mid-fight. Every single time. And when you ask why they do that, the answer is always along the lines of: "Because regen is annoying for pulling." Yeah sorry mate, but you aren't pulling NOW. I know how to time my regen, thanks, sir, and you are actually messing that timing up because now I have to recast it mid-fight with a full timer and it probably won't run out before the mob dies...