After sharpening lawn mower blades, balance them for best results before mounting them back on the lawn or riding mower. Balancing only applies to rotary blades, which you'll find on gasoline powered riding and push mowers. Reel type mowers have scissor-type blades which are self contained and virtually never go out of balance. Rotary blades, however, can become unbalanced from sharpening, hitting an obstruction or through normal wear and tear.
Pound a nail with your hammer into a wall, beam, framing piece of wood, or any other board that is perpendicular to the floor. Allow the nail to protrude approximately one inch outwards.
Place the blade onto the nail through its center mounting hole so that both ends are parallel to the ground.
Observe which end drops on one side or the other. The side that drops is the heavy side that you'll need to grind off.
Remove the blade form the nail, and grind off some of the end or along the original sharpened edge. Replace the blade back onto the nail, and watch which end sags. Repeat the grinding procedure until the blade stays parallel to the ground and no longer sags lower on either end.
Repeat this procedure with all of your lawn mower blades.