Excellent points doc and Skipster. Maintain your position Exact. Whether you elect to tell customers you have not seen it work or are a little more aggressive and show them research has to be up to you. I prefer to spend time and spread the word on what I believe does work. Having the grass as healthy as possible is your gig and dovetails nicely with helping maintain a degree of resistance.
That said, if you care for turf that is succeptible you will know it in Spring. If that happens, you can expect it again under the right conditions. It is a bigger problem for golf turf than lawns, in my experience. I use fungicide the first winter for turf that was new that Fall. That's it. Never had anything I could not grow out without much trouble.