My clemency rate goes up based on how inattentive the healer is. Perhaps there's a reason you keep encountering tanks that feel the need to use clemency so often?

My less snarky answer is that, since I play both classes, I have a feel for when the healer in the team isn't going to be able to heal all of us in a timely manner. Usually this is the result of someone else making a mistake, ie: DPS likes to dance around in AOE. Since I know the healer can't dual cast, I'll typically clemency an errant DPS because I know that most of the time the healer is going to pick me as his first priority when things go south.