My opinion is if your outdoor native soil is right for Mycorrhizae growth it will form whether you innoculate or not... If conditions are not right its not going to happen no matter what!

Certain fertilizers, chemical spraying, etc. or even too much water can effect Mycorrhizae growth.... That's my opinion...lol

http://cru.cahe.wsu.edu/CEPublications/FS269E/FS269E.pdf

I garden in beds so prep, fertilizer and/or treat the whole bed and not just the holes. I typically like to create a new bed a few months in advance of planting and that includes digging in compost and adding Planters II, (0-0-0) a "bio-mineral fertilizer" that is suppose to be super at promoting mycorrhizae. Then with perennials and bedding plants use an Espoma product (of the "tone" fertilizer) called Bio-tone Starter again with mycorrhizae at planting time. If I have some handy I'll use it when planting roses, although I tend to use more horse manure compost on the roses than anything else.

I agree with everything Jim said. Mycorrhizae is in the soil and will multiply on it's own if the soil is healthy. Not disturbing the already present mycorrhizae and other organisms in the soil is a good argument for not tilling or disturbing the soil any more than we have to when planting.

When I first started gardening, double digging was popular. I'm so glad we've learned that it's a terrible thing to do to the soil because it's hard work!!