Various people have stated that haste should be stacked to a comfortable amount, which is about as much as to bring HL and DL to 2 second cast. That's 20% haste. Through talents and buffs a paladin can get 12% haste on their own, 15% if in a group with a boomkin, warlock, etc. That means 5-8% haste needed.
I see a lot of people going for 10-11% completely forgetting they specced into Speed of Light and receive a passive 3% haste (does not show up on stat page).
I encourage paladins with this much haste to reforge it to crit until they hit the 8% (heroics) or 5% (raids). Remember, crit helps keep Conviction up while giving a higher chance of proccing Infusion of Light, not to mention more mana efficient heals.