Mycorrhiza are a group of fungi (soil microbes) that make nutrients more available to the plant as well as stimulating hair-root growth. I honestly don't know how they'd work in rockwool, it seems like there's nothing in there for them to live on as there would be with soil. If you put them into organic, plant-based components they'll likely break them down (myco inoculated soil ate through a big canvas cloth of mine in two weeks). They broke down a color-coded toothpick I was using on one of my clones, the colored part just fell over, the rest of the toothpick has been consumed by the soil microbes. That took two, three months.
If the seedlings are sprouted, then watch them for yellowing, I suppose. As I said, I've never used rockwool or grown hydro, but it just seems to me that those methods would mean you have to provide nutrients when there isn't soil to do it for you, so to speak.
