I have had 3 or 4 different watches in my hands (all SS) plus mine (SS as well) and on all of them the button had a slight play in it, even at my try-on appointment.
I think it's normal or at least within specifications. I also think that Apple will probably exchange no questions asked if it really bothers you. My first iPhone 5S had the same issue. I actually went through 3 units (2 of them because of other issues) and all but the last one had a loose power button. But they happily exchanged just for the button.
So, if it bothers you, make an appointment and see what they say. As far as I can tell, even an exchange might yield the same result.