If your going to use Hocking Hills SP then a reservation is a must as that is the number one rated State Park in Ohio I do 99% of my camping with a sunday after 3pm check in and a Wednesday 11am checkout. Reservations are not really necessary under these conditions. Call the park in advance (2 weeks or so) and ask them about their occupancy on the days that you want to camp. I have never had a problem with them telling me that on Friday to Sunday they normally only have around 95% occupancy. If you must have electric or full hook ups then a reservation is a must.