Size of
sign could dictate how to appraoch this.
I'm a firm believer in nothing [
any signs] going right up against a wall, unless it's a little no parking
sign. We almost always use some sort of battens and then you don't hafta be 100% exact putting them on and can use your good eye to use a strong exterior grade wood screw and either countersink it or use a decorative type screw. That would depend on the type
sign and how it's being used.