Long time gardener here with a wide variety of flowering plants in my garden which would not get sprayed with anything that would harm them. I do spray in early spring when the quack grass starts growing and the perennials are still quite small being careful not get the spray on them if possible. Using a hand held sprayer allows me to better direct the spray to a limited area. I know that some small amount of spray sometimes gets on my perennials, so far have not lost a one of them due to the spray.
Each person should absolutely do research and ask other gardeners in your area how they handle the quack grass problem. I also got information on using the fusilade II spray from members of the AHS since besides my perennials I also have plenty invested in daylilies wanting to cause absolutely no harm to them, am a longtime member of the AHS society. I would also recommend doing some test areas to see if whatever you choose works well for you.
