SATURDAY, FEBRUARY 23When you're a musical icon like K.D. Lang, there's no pressing reason to endure the hassles of touring. If we were in her shoes, we'd be sitting back, living off royalties ("Constant Craving" alone must be worth boatloads), and recording the odd charitable single or duet with a famous friend whenever we needed to shake things up. But the Grammy Award-winner is traveling the world in support of her new album, Watershed, a self-produced collection of original tracks. Lang appears at the Opera House (539 Washington Street, Boston, 617.880.2442) tonight at 7:30 p.m. Tickets are $43.50 to $63.50; get them at www.ticketmaster.com or 617.931.2000. @