I had my fabric on boards and then switched to ruler folded. I do agree that vertical on boards is easier to get to a particular fabric - but only if your shelves are not very deep. My problem was that I bought deep cabinets (one is 18 in. deep and the other is 23 inches deep), so that I could also keep bins of UFO projects in the cabinet. My husband wanted everything hidden behind doors/drawers since my main sewing room is also the living room. with my fabric vertical on boards I had to have several layers on 1 shelf - eliminating the 'ease' of access. Also my shelves could not be adjusted evenly so that all stood exactly vertical. If I arranged them with enough room for true vertical usage then I actually did not have room for one of my shelves.
Because my cabinets are deep flat ruler fold is a more econmical use of space.