It doesn't matter, as long as it's convenient for you to use.
One consideration that does matter, though, is what type of wall you are mounting to. Generally speaking, you want to use a masonry wall, which would typically be found along the outside perimeter of most homes.
One consideration that does matter, though, is what type of wall you are mounting to. Generally speaking, you want to use a masonry wall, which would typically be found along the outside perimeter of most homes.