Magnetic or not depends on the alloy.

[Link: fastenal.com]

Which brings up another point. The exact color tone of the "stainless" depends on the underlying alloy and how it is finished. For example, is the surface "brushed" and if so, how fine is this "brush"? Depending on the proximity of other "stainless" fixtures and the lighting, this may not be overly important, but this should all be considered and specified by the designer.

I think that "magnetic" or not is the least of your worries. In the end small magnetic pads or strips could be attached to the back of the grill if necessary.

[Edited typo]