Unless you have a completely sterilizied soil mix, i don't think it would be beneficial to add mycorrhizal fungus. It's almost like growing shrooms though, the more spores and things you put in, the more likely you are to have it. If you are interested there are two types, ectomycorrhizae which develop on evergreen trees and shrubs, and endomycorrhizae which was formerly known as vesicular-arbuscular mycorrhizae. Most nurseries that raise trees for orchards or for forests use the second type, so I'm sure that's the kind you want for your babies. Also, these are the type that put their hyphae into the cell wall and are the "better kind" at least for our particular application.