Alex Scoles boats have the central mainsheet jammer about 10cm above the floor of the boat e.g. on a piece of aluminium or carbon. Don’t take any notice of these guys that don’t use centre jammers. Dave is right that the base needs to be angled back a bit so you can play the main on reaches without it dropping into the cleat. It is entirely possible to get the angle so that the main goes into the cleat when sitting out on the beat, and releases reasonably easily. I always keep the mainsheet in my hand even though it is jammed in the cleat – just in case!!
If you can learn to use it properly a centre jammer does result in your tacking becoming much smoother. The top of the fleet is pretty well divided 70:30 in terms of using a centre jammer v cleats on the inner edge of the side deck.