The purpose of pre-soaking rockwool is to flush out the alkaline residue left behind by the manufacturing process.
The pH of the soak solution doesn't matter too much, but the lower the pH the faster the deposits will be dissolved. pH < 5 breaks down rockwoool and should be avoided. At pH 5.5 you only need to soak for a couple of hours, at pH 7 or higher you should soak overnight.
After soaking, flush it out again with your starting solution @ pH 5.8.
If you are starting seeds you can use plain water @ pH 5.8.