Personally the only reason I think people soak the cubes is to let the PH kind of level out, which to me does nothing. I like to saturate them with PH'd distilled water then sort of fling them out until they are just damp then I just plug my seed in. Once the seed has sprouted I put the cube in a cup with 2 paper towels folded up on the bottom which is just to the point of saturation with tap water PH'd to 5.8. Over the course of the next few days I keep the paper towel saturated and will once dip the bottom portion of the cube in a saucer with a small amount of PH'd water just slightly wetting the very bottom. Usually by this time the tap root is reaching out of the rock wool ever so slightly, it's at this time that I move it into my DWC setup, I keep the water one fingers width from my net pots bottom, fill my net pots with one to two layers of hydroton then place cube in the pot. Once I have roots well established I slowly move my water level down to a full hands width from the bottom of the net pot.
It is very easy to over water in rock wool but for me the trick is to either weigh the cube dry then weigh it after you water which will generally tell you how much water the cube is holding (thanks al b fuct), or provide some media that it can wick moisture from temporarily while rooting. Just remember not to leave it too long on the paper towel as it will eventually start to breed bad bacteria which is a no no in DWC. If you go longer with the paper towel just change it out. I hope my experience gives you a little more insight on getting started with DWC, good luck and happy growing.
KnB