Well kind of. The load isn't constant per se but without the engine running there should be very little free play. That's what the tensioner(s) are for. If the tensioner(s) are at max length and there is still free play in the system then something is pretty wrong. The tensioners are designed to take up any slack in the system and keep the loads on the belt relatively constant - that is, when there are extra stresses put on the belt for whatever reason, the tensioner is designed to absorb some of that force and also to provide force when there isn't enough stress. I'm not familiar with belts as I work with chain systems for a living, but I'd assume they're the same.