Much depends on the available natural inoculum. Lots of papers compare mycorrhizal plants with non-mycorrhizal controls (same for bacteria). This is not a lot of use in crop production unless (as, say in glasshouses) the substrate is sterilised. You need to run some trials with your inoculum comparing with whatever is available naturally before making up your mind. You also have to consider the 'bottom line'. It isn't much use in spending financial input on a crop if the resultant (if any) yield increase doesn't more than pay for itself.   _{Your Comment}