There was an article in the New York Times on Saturday touting the Big 4's success with flex time.
You should read it. The title is, "Flex Time Flourishes in the Accounting Industry."  Of course, it showcases the Big 4 and a couple of other huge firms.
Local and regional firms have been offering flex time for years (dare I say decades?). The firm I was affiliated with for 30 years has offered flex time for 25 years. I rarely talk with a firm who DOES NOT offer flex time. Wonder why they didn't talk to any local and regional firms?
If you don't already offer flex time, it is a way to keep the best and brightest people for the long-haul. It is pretty much the norm for CPA firms now.
There are lots of different ways to structure your flex program and to manage it. Do you research and make sure it is a part of your firm's culture.