There are concerns with aluminum and stainless steel being in direct contact with each other. Think of it as a small battery
. Aluminum by nature acts as an anode. Stainless steel acts as a cathode. You need an electrolyte solution to get current flowing. In this case rain water could act as the electrolyte. If you live close to the ocean, salt water can act as a very good electrolyte. The current that is generated is what causes the corrosion.
Fortunately, if you have a larger anode (the aluminum window frame) and cathode (the SST screws) the concern is still there, but, is very, very minimally less.
If you were to attach a large SST metal sheet to something outside using aluminum rivets or screws, the rivets or screws would corrode very fast, especially if you lived by the ocean.
The washers would limit the contact area between the frame and the screws and would certainly be a cheap, easy solution for peace of mind. I really don't think it is that much or a concern or necessary.
Just my 2 cents worth