Cover as many sides as you like, the background is all asthetic and personal taste
If the tank is empty, paint a thin coat of cooking oil over the outside bits to be covered, and lay on the background. Smooth out any large air bubbles with a bank card or other suitable flat and flexible impliment before leaving if for a couple of days. By doing this, the backing will stick without any visible airbubbles in it, and will give a nice clear and bright finish to the background. Trim the excess off with a sharp craft knife and rince the tank to remove any excess cooking oil before filling for use
If the tank is full, tape it on
If you have the internal backings, thoughly clean the tank with a sponge, and methelated spitits on a cloth. Leave after the meths wipe-down for a few moments, before applying the silicone to the background in the manufacturer's specifyed places and press firmly onto the back (and/or side) glass(es). Leave the silicone to set for at least 48 hours before again rincing and filling for use

You must use aquarium silicone for internal backings, as building trade silicone will leach toxins into the tank
HTH
Rabbut