Never heard of that method before. Seems odd to me, but what do I know? I can see that the dampened leather would probably cut easier, but I wonder if there's any amount of swelling due to the moisture, then shrinkage later as it dries. Then again, we're just talking a tiny bit of water, not dunking it in the bath tub.
Are you using a lathe with a tailstock? If so, put a concave live center in the tailstock and place it against the tip with a bit if pressure, then trim the tip. The live center assures that the tip cannot flex away from the cutter, so the tip is trimmed exactly straight with the ferrule. As you get close to the ferrule, start taking very small cuts. You almost won't have to clean things up with fine sandpaper, depending on the tip cuz some cut much nicer than others. Finally, a carbide cutter will go thru any tip like butter.
I don't mean to insult you. I don't know who you are, what your skill level is, what kind of machelinery you have, etc.