I think when you add it also depends on what you'd like to get out of the molasses. Molasses is going to contribute to the gravity of the wort, flavor, aroma, and color of the finished beer. It's going to contribute to the gravity whenever you add it to the boil. But, flavor, aroma, and finished color are all going to change a bit based on when you add the molasses. You'll drive off aroma compounds with a long boil, and you'll contribute more to the color than with a short boil. If you want more molasses aroma and flavor, add it near the end. As others have said, when you add it affects your hops utilization, and you'll want to account for that.
As with dry yeast, make sure you've cooled your wort to <80° before pitching your yeast.