Their should be no problems with any type of markers, other than steel rebar. I have hit some obstructions in the past, moved the marker over an inch or two, and back in buisness. Thick wood markers (over 3/4" squared) might pose potential problems as well!
Joel you do have to use care when installing any type of marker that requires you to pound into or make a hole before you install. Not only due to sprinklers but irrigation and drainage piping but low voltage lighting wires too that can be burried along the drives and walks. I would "ask" the customer it they know of anything in and around the area that you plan to mark, at least it would show your concern to the customer.