I did a little homework on wheels when I made my steady a couple years ago - the durometer rating indicates the hardness. There are two scales A and B. The A scale is most common, with numbers that range from 78 to a little over 100. Larger number, harder wheel. The B scale is measured differently, so an 80B wheel will be harder than a 90A wheel (the difference between the scales is about 20).
All this and what I found for my equipment (AB lathe with 24” homebuilt steel steady) is that harder wheels actually resulted in less vibration in use compared to wheels on the softer side. I think that since the softer wheels tend to absorb some shock that having three on the same plane set up some harmonics (just a guess). Harder wheels solved the problem for me...also, white or clear wheels leave fewr marks. If I remember, I try to leave a slightly raised area for the wheels to track that gets taken down in final sanding. When I forget, a single layer of masking tape helps prevent any marking, but it hasn't really been a problem.