Hi John, I've heard of people deliberately mowing peonies down after they bloom with no injury to the plant. They do build up energy during the period after blooming until the leaves are killed off by weather in the fall so you'll lose that boost to the roots, probably won't increase as much as they would if left longer, but no ultimate damage. Will you be able to completely shield the remaining stems from your spray? As long as they're shielded and you don't use a spray that can damage their roots you should be fine.

On a personal experience note--I don't spray but I've had several peonies look bad enough during the summer that I go ahead and cut them back early, they've always come back the next spring.

I do have another question: I live in Zone 5A in Ohio My question is do peonies do better in Sun or in shade? My peonies are in sun until 10 in the morning and shade in the rest of the day.

Almost all herbaceous peonies do better in full sun, 6-8 hours per day. There is sometimes advice given with the darker reds to plant in some shade to keep the color darker. A lot of tree peonies can take more shade with no issues. And there are some "woodland" species peonies that actually prefer shade.

The hotter your summer the more shade your peonies actually prefer so growing in dappled or some shade is better in that case. As long as your peony gets a few hours of direct sunlight/day it'll grow, it just might have leggier, more sparse foliage and fewer blooms.