Been thinking about this for the past few days - how much of a difference does the cascade make in this process?
My answer to your question is very subjective. All I can say is how I got there along with some scientific mumbo jumbo and my personal observations.
I started out using tap water with a drop of Dawn dishwashing soap. This is something that I picked up many years ago when I spent some time in auto body shops. The idea is that a little bit of soap lowers the surface tension of the water which helps wet the paper and and the surface being sanded which in turn helps the the slurry (water, soap, and removed material) flow more thoroughly therefore reducing scuffing.
The Scientific Mumbo Jumbo: Dish soaps like Dawn contain Surfactants. Surfactants are chemical compounds that reduce surface tension of water which helps wash away oil and grease. Most dish soaps, as well as laundry soaps, hand soap and body washes contain Anionic or negatively charged surfactants. They are widely produced and are made from a range of raw fats and oils like soybean, palm, tallow and coconut.
A couple of years ago, I took this one step farther by switching to a drop or two of a nonionic surfactant instead of "dish soap". It reduces the surface tension of the water more than dish soap and it not only helps wet the paper and the blank more thoroughly, it also helps de-wet the surface when the slurry is being removed, especially when wiping the blank with paper towel following each grit. The most conspicuous observation is that blanks start to de-wet when I get about half way through the MicroMesh grits. It is an obvious indication regarding the surface tension of the liquid. In addition, although it is entirely subjective, I think the level of gloss is higher than when I was just using the dish soap.
More Mumbo Jumbo: Nonionic surfactants contain no charge which makes them less likely to form a "soap scum" in hard water. As for cleaning ability they are much less effective than anionic surfactants so they are marketed as a rinsing or drying aids rather than soaps.
So, in my very subjective opinion, yes, they make a difference in the glossiness / luster of finished blanks. - Dave