I think that as long as she works and tries to grow as an actress, making sexy shoots for magazines won't really hurt her. Look at Angelina Jolie, probably considered the biggest sex symbol, yet she managed to build a career aside from that image.
Ash is a sexy girl, so it's only logical that magazines are going to ask her to do sexier shoots. Plus she's relatively new to the business. Twilight was her first film and she's been working nonstop since then. If all she was doing was posing naked, then I'd understand. But she's not.