Toe in is probably unnecessary on brakes that have stiff brake arms and pivots with no play (like dual pivot calipers), but is mostly necessary for cantis and V brakes because of flexy brake arms and sloppy pivots. When the pad hits the rim, it puts a torque on the brake arm which causes the brake arm to twist. Instead of hitting the rim flat, the center of pressure will be towards the rear of the pad. This causes the rear of the pad to wear faster than the front. It also helps eliminate pad squeal on these brakes as well. So, I would toe in pads on cantis and V brakes to get uniform pad wear and quiet brakes.