I'm inclined to agree with Larry. You may get some benefit in soilless mixes (I would try a test treatment and keep some untreated to see if there's a difference over time) but less likely in the garden soil. These fungi occur naturally in soil and are adapted to your local environment and plants. Unless the soil is very poor my understanding is that you may see no benefit from adding a commercial product. I recall one hybridizer telling me he'd tried mycorrhizal innoculants for daylilies (I presume planted rather than potted) and saw no difference.
From a report prepared in the UK about the beneficial effects of mycorrhizae are these notes at the bottom of the article, paraphrased as the iPad won't let me copy direct:
Transition from demonstrating beneficial effects in the field as opposed to lab have been difficult. Conventional systems have little gain as the fungus drains carbon from the plant whereas the benefits to the plant from the fungus can be achieved less expensively with fertilizer and biocides.
They comment that there hasn't been a high demand for commercial product perhaps due to unconvincing literature concerning efficacy in field situatons. Successful field trials are rarely found in journals and commercial ads don't provide evidence of benefit in field conditions, only giving examples fom controlled lab and greenhouse conditions.
This is from:
I have no idea if the fungi that colonize daylilies in nature are included in commercial products, or how much daylilies might benefit from them. There's very little info available on mycorrhizae in daylilies. It would be interesting to do a test with daylilies, one group given mycorrhizae, one given fertilizer and one group untreated. The few commercial products I've looked at also contain fertilizer which makes it difficult to separate the reason any apparent benefits.
Edited to add this link to an article on mycorrhizae by Dr. Linda Chalker-Scott of Washington State Univesity: