I do a finish grind on the spring/blade once the knife is assembled. I leave extra meat on the top of the spring. All through the fitting, the spring is proud of the blade. Once I'm happy with the blade angle in the open position, I go ahead and grind the spring down flush with the blade. If the face of the spring is fairly flat to the back of the blade (the spring face's mating surface) the butt will be darn near invisible when the knife is open.
If you're getting an obvious line between the two, then it could be that the angle between the spring face and the blade aren't flush enough. Now, having said that- my way prevents you from breaking the corner edge of the blade very much at all, or else that broken edge will look like a line when the knife is open. I do break the edge, but ever so slightly. Usually a buffer is enough right on that corner.