The greater the DIFFERENCE in how much more noble or less noble a metal in comparison to each other determines galv corrosion. In other words, the highest grade screws used on aluminum won't cause problems. They are both more to the noble end of the scale. By the way, they usually don't list what grade the ss screws are when you go to buy them. So don't get screwed by not bringing your magnet. So quit tryin' to throw things off the magnet. It works.:B
I can get completely non magnetic (highest grade) fasteners any day of the week. They cost more and are used without isolation washers on aluminum boats all the time.