Tom,
You don't mention how wide (long) the shelves will be and what kind of load they will have. If you are planning to load it up with hard cover books or something else relatively heavy, you should consider using the edging to add some strength and prevent bowing. One simple way I've done this in the past is to cut a piece of solid edge material about 1 1/2" wide (you can use what ever width you want though as long as it's wider than the plywood is thick). Then cut a 3/8" high by 3/4"+ rabbet on the back side. Basically an "L" from and end view perspective. This will be the surface that you glue to the edge of the plywood. You can just clamp it since you have a lot of glue surface and won't need to use fasteners. Come back after it dries with a flush trim bit as mentioned above to trim off the little bit that is sticking out above the top surface of the shelf. This adds strength and visual mass which might also enhance the appearance of your project.
Good Luck.