The pH of the rockwool is only really important when you are first starting and planting the seed/clone. As you condition it (run your system), the pH will level out. The best idea is to soak them overnight in 5.5 water, then put them in your system and run it for a day like stinky said and see what happens with the pH. If its way out of whack, keep running it until it levels off. Eventually, the rockwool won't have much effect on your pH levels. It's overwhelming to start, but once you get the hang of it, its pretty simple really. Good luck!
The above post is fictional and should in no way be taken as truth.
If I were to grow cannabis, I would only do so to supply myself and my close friends. Doing this would take myself and my friends out of harms way by knowing the source of our cannabis, as well as take hundreds of thousands of dollars off the black market annually. It would also lower the demand of black market cannabis, and would ultimately help keep cannabis out of the hands of minors.