Apple suggests using a case if you are concerned about this. From the bottom of https://www.apple.com/iphone-7/:
"The high-gloss finish of the jet black iPhone 7 is achieved through a precision nine-step anodization and polishing process. Its surface is equally as hard as other anodized Apple products; however, its high shine may show fine micro-abrasions with use. If you are concerned about this, we suggest you use one of the many cases available to protect your iPhone."
I imagine an ill-fitting hard case with minor imperfections on the surface in contact with your phone may also scratch it if the phone slides around in it slightly, so you may want to keep this in mind when choosing a case.