I would not unless there was some other reason. The weight of the concrete blocks is not nearly as heavy and would not work to hold things nearly as well. But then without tanks on top of the boards, what would one use them for? A place to hang lights, maybe? For something pretty light and easy to move, etc. like lights, I would just lay a 2X across if I did want to hang lights. The weight of the lights is not likely to bow any solid stuff and if it did I would not find it much trouble to change without having tanks and such to deal with. For a rack to hang things from, one easy way to make sure wood doesn't bow is to add a second piece at an angle to the first making an "L" like an angle iron. Flat stuff like steel bar will bend easy but add and angle and it is much stiffer. To really go for stiff making a "C" does more. Reason for "I" beams in buildings? Super duty strong.