Before pull
This is also getting a huge buff soon and will no longer cause an increase in damage taken. Rage starvation really isn't a huge issue during a fight
I don't tank much on my druid these days since we usually have regular tanks and i'm playing my hunter, but i rarely have issues with rage, on some occasions in heroic 5mans but rarely. Sorry lets hope someone else posts who knows what they are talking about
Carrying over rage between pulls depends on how much you have left at the end of the last pack and the time period before the next pull (healer mana dps afk etc)
In heroics I just use it whenever and then pop a defensive cooldown (usually barkskin) to offset the damage increase. I LIKE to use it before pulls, but if it's on cooldown, I just get my furor rage and go in, popping enrage once it's ready. I'll avoid using it if I'm tanking raids (which I haven't done in a million years) since you barely ever need rage anyway when a boss is hitting you for large chunks of damage.
If its off cd I use it pre-pull. A simple right click of the aura cancels the incoming damage increase or u can make a macro to use /cast enrage then on the next line /cancelaura enrage. Youll have to hit the macro twice but its handy if u need to use enrage mid boss fight