Once you acquire a design patent, you must mark your design work with your design patent number. (You’ll receive this number when the USPTO grants your patent.) Any placement is suitable, as long as the number can be located by an ordinary user.
Failing to include the notice could cost you money if you later sue an infringer—even if you win. For example, when the Nike company sued Wal-Mart for design patent infringement, the court ruled that Nike was not entitled to a portion of Wal-Mart’s profits or statutory damages (damages fixed by law) if Wal-Mart could prove that Nike had failed to mark the design patent number on one of its shoe designs. (Nike, Inc. v. Wal-Mart Stores, 138 F.3d 1437 (E.D. Va. 1998).)