ive never used rockwool. i think rockwool is a horror story just waiting to happen. maybe its my inexperience with the stuff but i have read lots of negative results using rockwool. i use perlite to start my seeds with. take a plastic cup and put some holes in the bottom on the sides. fill with perlite and sit the cup in a dish with 1/4" water. the perlite acts as a wick to pull water to the plant.
some ppl have good success with rockwool tho. just not me.
It's not the best thing to germinate seeds in rockwool, it does work but you will have a less percentage of germination using this method. It's better to use the paper towel method or what Weeddog suggests.