Side mounting is superior to an upright engine in a number of ways with the most important being fuel delivery. Its is much easier to get the fuel tank spraybar relationship in the proper position. Also in most cases the needle sticking straight up is a much safer senario. In the case of a taildragger with a stock muffler this muffler will also serve as some rollover protection and in some designs can actually serve a brake by application of down elevator, Yes its common for some activities.

John