I Have made a few, and what I have noticed more then anything is that the pressing, while drying, and sqeezing the moisture out, is the biggest key to getting them compressed well, if that's what someone wanted. The liquid, providing It does not rot the tip, mostly just seems to help with the tip holding It's shape after pressing and drying out. Hot water or steam may do the same thing but faster if It pentrated the tips interior better by some chance. Wood craftsmen have been doing the same thing to bend woods for a very long time. Like simular to wood bending- You can put a dry spongy tip in a vice, and when you pull It out It may only spring back to where It was originally(some more then others), and only time & use will compress It anywhere near fully seated with some tips, but if you soak that same tip, press, and hold until the tip dries out, It may hold It's shape much better Like wood does. I'm sure some liquids may do a better job then others based on what properties It has in It's formula, but for the most part they are all doing the same thing IMO.
With that said, while experiementing for the heck of It, I have used milk in the fridge, and out of the fridge, water, even mineral spirits believe It or not, and they all did basically the same thing. I'm sure you could use colas or root beers also of ya wanted

, because the princible is basically the same. I don't really know which works best though, or if certain liquids will give you a different hardness level then others, because I have'nt used any methods of gauging My clamping pressure other then By feel or thickness, and the comparison would be flawed by the differences in each attempt. I have no way of testing the tip hardness anyhow. Basically I can see no noticable differences either. I have noticed some are sinkers and some are floaters, I don't know if that makes any difference or not. Possible that less time in a vice could produce a less compressed tip with a softer hardness level if that was desired instead of a firmer tip.
Greg