I have never been around a place where people dig their own and usually the row width is for mowing the grass or plowing the soil. It depends how large you are going to grow and sell them at depends on the spacing. For allowing people to dig their own I would allow more on center. If you are going to dig 7 feet will be okay depending on the variety and allow more room for shade trees.

Here is a link that might be useful: Propagating Perennials

In my county we have a large nursery industry that includes 6000 acres of field planted trees. Farmers won't rent land to nursery companies because when they harvest 1000-2000 or more trees per acre each one with a 2ft root ball they deplete the topsoil leaving the land worthless for farming. Keep that in mind if you plan to grow field planted trees. Space as wide as possible or plan ahead for a harvest date if you plant closer. I can show you acres of worthless trees where the trees grew into one another...Pot in a Pot is the way to go... Bob
Bridgeton NJ.