I know we've all decided that we prefer the tone of nickel silver to brass, but neither interferes with the magnetic field directly since they aren't ferrous metals.
It's really about the conductivity of the cover and eddy currents that flow on the surface.
Also nickel silver is a form of white brass, so I'm wondering how different they really are as far as conductivity.
Now if one cover is thicker than another, that would make a difference. Also if they are plated, which they all are, having a thin copper layer might make a difference.