what thickness plywood for storage shelves?
At only 6-8 inches wide, I would truely consider solid lumber shelves. Plywood would sag for sure, eventually, with any load at all.
If you are intent on use plywood, at least make a solid edging piece to stiffen the ply along its length. I recently made an inset cherry shelf unit for a client, using 1/2" cherry plywood to span the 28", but milled a solid cherry nosing for the front edge of the plywood. This greatly stiffens the shelf and helps to prevent sag. Although the edging I make are usually 'L' shaped in cross-section, even a solid strip glued and/or nailed to edge will also help.