The method used depends on how many high spots and how bad they are. It also depends on whether the longest tapcons will bite with the spacers required. Usually you can hit the high spots with a sledge and a coal chisel to even it out but you will probably still need spacers. Depending on the severity, you can stack fender washers or use tubing cut to length. If it is really severe you may need lag bolts and lead anchors and tubing. All in all it is a judgment call based on what you charged for the install, what is best for the customer, sign and building (customer is a tenant or owner), and the severity of the wall.