The reverser will let you center the servo in relation to the other side but it can introduce throw problems that have to be corrected mechanically. With both elevators centered look at each of the servo horns, they should both be at the same angle in relation to the servo case. If by centering the elevators one of the servo horns is at a right angle and the other is not then the throws will be off. If both servo horns are at the same angle then check the elevator control horns to make sure they are mounted with the clevis pin holes the same distance from the hinge pin centerline. If either elevator control horn is further forward or further aft than the other you will get unequal throw.
Remember that a mutli arm servo horn can be rotated to find the arm that come the closest to the angle you need reducing the amount of adjustment required by the servo reverser.