My 5S power button wiggles a bit when I shake it. Noticed it immediately.
Then picked up my wife's 4S, my old 4S, and my mom's 4. All rattle a little bit.
Really not the end of the world ... my 5S is pristine, has a flawless screen, and no other defects. Could spend months chasing a 5S with a power button that doesn't wiggle slightly and still end up trading that for another problem.