The best and safest cleaner for a mirror is clean, warm water used with a soft cloth. Be careful not to allow the edges of the mirror to get or remain wet over a period of time.
Do not use any acid or alkali cleaners for mirror cleanup after installation. They can attack both the surface and edges as well as the backing of the mirror. And never use an abrasive cleaner on any mirror surface.
Use a standard brand of glass or mirror cleaner that does not contain ammonia or vinegar.
Remove surface marks or stubborn dirt with oil free steel wool, not solvents. These could attack and damage the edges and backing.
Never spray any cleaner directly on to a mirror. Instead apply the cleaner to a soft cloth and then wipe the mirror.
Always use soft, grit free cloths when cleaning a mirror to reduce chances of scratching the surface.
Be sure to dry all joints and edges thoroughly to be certain no commercial cleaner comes in contact with the edge and backing.