As a fellow midwesterner (illinois), I surmise that you are well past petal fall,
so your question is somewhat academic, at least for this season. I have
never applied more than one spraying of dormant oil, usually in the late winter or very early spring. I know that people spray even in the summer
with a diluted dormant oil. The danger there is burning of the leaves in the
hot sun. Unless you have special problems with insects that lay their eggs
late in the season, I see no benefit of the additional dormant oil spraying.
You will find benefit by going on a regular spray schedule of fungicides and
insecticides for the remainder of the year. This depends on what and how
much are the attacks on your trees.

Sherwin D.