My guess is, as duke37 noted, brass screws are more desirable because they don't corrode. That's my guess as
to why whomever you got the switches from are missing the brass screws. They were salvaged because whomever took them wanted them for his own use in something else.
In my younger days I started out as a residential electrician. At that time aluminum wire was coming into use because
it's cheaper than copper wire.. The problem that my boss explained to me, was that the service entrance buss bars were still copper. Mating the
aluminum wire to the copper buss bars somehow caused (with connections that weren't tight), the aluminum wire to vibrate with the 60Hz on the power line, resulting in a fire. I don't know how accurate that explanation was, but I
do know we accidentally burned down a few houses back them because to the aluminum/copper connections at the service entrance.
Nobody ever explained to me why, but like you, I noticed on the outlet and switches themselves, one steel screw, and one brass screw. I ASSUMED they did that so you would know to wire the neutral to the steel screw, and the hot wire to the brass screw, just so the entire house would be consistent. All the neutral wires to steel, and all the hot wires to brass. Once again, I just assumed that, and somebody here might have a technically more accurate
reasoning.
Because of the return of mostly copper wire, I don't THINK it matters if you replace the missing brass screws with
steel ones, they've always used steel on the outlets and switch neutral wires. I can't think of a reason why they both couldn't be steel. UNLESS there is actually an electrical code that specifies brass be used for hot wires. Call an
electrical supply store and ask about those screws.
I will mention however, if the electrical supply store says steel for both is ok, that you should take care to ensure that, for safety sake, you consistently wire all of the outlet connections exactly the same. All the polarized plug neutrals to the white wire, and all of the hot sides of the outlets to the black wires. (Which is why I THINK they used steel and brass screws, to identify one from the other).
That's my 2-cents worth.