When I needed to replace my header nuts I went for stainless, but also bought domed headed ones in stainless as well, also known as acorn nuts. The idea being that once the studs have been copper slipped and the bolts tightened, the exposed thread is covered by the domed nuts purely to protect the exposed area. Once in a while you could slacken off the domed nuts, reapply some copper slip and retighten.