It really depends on the situation at hand really.
In raids? Absolutely, regen at 5-6 seconds so I'm ready to go full tilt from 3.
On pulls in dungeons? I look at a few factors:
If the tank is a PLD, more often than not I will regen after the first pack has been flashed on the pull, it can add up to a pretty big sum over the course of a long pull and most PLDs don't really have an issue with it. Same goes for DRKs usually. For Warriors I usually avoid regen and save my agro headroom for dotting up on the pull instead. There's also their preference to take into account as well. Some are happy to let me get away with it, some are really not so keen.
In short, if I can get away with it without annoying the tank or making a mess of the pull, I'll regen once the first pack is tagged and my dots are ticking. Getting regen going before the pull comes to a halt gets me an extra holy right off the bat so it's worthwhile if it doesn't cause issues IMHO.
https://www.youtube.com/watch?v=fdU4tRrX984 pretty much demonstrates how I work. (except pretend the regen I cast on myself was meant for the tank, oops!)