im a contractor my self... and i know we use a barrier between anything that touchs concrete yes, but riddle me this, i have a untreated/unfinished stand for my fish tank in my basment for years, and im not the only one.... my father built his stand 20- 25 years ago its untreated and has been sitting on a basement floor in the same spot with no barrier,
i think you would be just fine with out treated, now with that said if it was me and i was going to build it i would use treated on the floor the price is not going to kill the pocket book for 1-2 extra boards and it peace of mind,
and also the comment about the chemicals in the treated wood, i went to a siminar where a scientice explained that burning the wood is the worest release of the chemicals in the lumber, but for it to even be some what harmful for you you would have to burn way to much of it, dont remmember the numbers... and that you would most likly die from the smoke before you would the chemicals