It all depends on the type of rack you want to build. If you want more of a shelving unit, then you should leave the lids on (or if you, like me, are placing a tub on the top shelf of a rack). But, if you intend to build a typical rack with the basic 1/8" spacing then you leave the lids off and simply slide the tubs. This 'drawer' rack requires more precision in measurements and caution in building, but it is nice to just be able to slide the tub out an inch or so to peek in for messes.