Blessing of Salvation is a feral druid's best friend. When we're
DPSing it keeps us from pulling aggro. And when we're tanking it makes
holding aggro EZ-mode for us. 30% threat reduction for all your
smack-happy fire mages and destro locks is great.
Blessing of Salvation on feral druids is also a paladin's nightmare. Sometimes we're going cat DPS and need salvation. Sometimes we're tanking and salvation can mean the raid wipes. Which is it, pull by pull? The poor Paladin has to keep track. Worse, Paladins choose their buffs on a per-class basis. So of course they're giving Salv to all the trees and chickens and it's easy for them to forget to replace Salv with Might or whatever after rebuffing.
My general rule is I only take Blessing of Salvation when I'm DPSing a boss. On trash even if I'm not tanking I always wear a tank set and don't take salvation; it's far more useful to suddenly be able to tank than to do 30% more DPS. (And Cower is remarkably useful at the start of a pull if you are risking pulling aggro.)
I try my best to be sure I don't have Salvation when I'm tanking, but in my UI it's just one more little icon to track. What I need is a BIG RED BOX on my screen when I'm in bear form with salvation on.
Update: thanks to Faerun for pointing me to the Damnation addon. It automatically cancels Salvation when you're in bear form, just the thing I needed.