Garter stitch is generally a flat stitch, without the tendency to curl that stockinette stitch has. Has the sweater been blocked? Blocking will set the fibers in the shape they've been knit into -- it's basically a gentle iron with lots of steam, and then keep the garment flat until it dries. This should be easy with garter stitch but even stockinette will lie flat once it's blocked. This assumes, of course, that you're not talking about a sweater that fits so tightly the buttons pull the band when it's worn, or anything like that.