Should the screws and covers be made wet with a salty solution or any electrolyte you would have a problem. It may not apply to you with the salt nor I as it never sees the winter but for others it may well be a bad idea.
If they were tightened too much the threads in the covers would tear out.
If you want to use stainless screws get ones that are plated.
Play it safe, get aluminum screws.

My 300 has aluminum covers behind the brakes and they were held on with steel screws, the salt attacked the aluminum around the screw head and ate through it leaving aluminum washers under the screw heads, they later went back to steel shields.
Galvanic corrosion is what caused the problem, another fine example of Chrysler engineering that was probably dictated by some ignorant accountant who was paid to save money.