Typically signs produced using light gauge material, .024 plastic and .012 aluminum do not need to have holes drilled, as the material is so easy to nail or drill right through.
With the heavier gauge custom signs it will depend on whether or not the signs are going to be hung the same way every time. Some customers decide not to have us drill the holes because they may hang them in many different ways. They find that it is easier to drill the holes themselves so they can drill them in different places depending on how they are being mounted.
Some of our stock signs come without drilled holes for this reason. Ohter stock signs have holes drilled at the most commonly used location for that particular type of sign.
However, if you are ordering a custom sign and know that you are going to be mounting the signs all the same way then it may make more sense for us to drill the holes for you.