It's a cardinal rule in show business that one should never perform with children and animals. There were no children onstage at the Roberts Orpheum Theater the night in April when Steve Martin and the Steep Canyon Rangers came to town, but there was Wally, Martin's yellow Labrador. Wally wandered onstage briefly during sound check to survey the audience, but his greatest moment came during Martin and the band's performance of "The Crow," a rollicking bluegrass number and the title track of their latest album. Just as the song reached high gear, Wally moseyed to the middle of the stage, checked out the audience again just to make sure he was the center of attention, let out a huge yawn, then turned around and curled up at Martin's feet. It brought the house down. Even Martin laughed, though he kept picking away at his banjo without missing a note.