A Concrete floor is important, if you want to be efficient and have more than 1 guy load. Inexperienced operators tend to dig down rather than pull in and curl up. Our mulch bins are on a concrete pad, and we keep a couple of floater bins there - one for topsoil in the summer, sand/salt in the winter, and 1 has crushed granite in it now, but might have belgium block, or other at any time. We keep stone on the ground without a concrete pad - basically I got tired of dropping $1000 a load for concrete. You have to be a little careful for clean drainage stone, but after a bunch of loads you sort of end up working off of a stone base. Same thing for the processed and dust. For topsoil, if we don't load the bin, and just keep it piled at the screener belt, we end up scraping up the base of the pile every now and again to run it through the trommel again. There are just so many times you can drive on soil before it packs hard.
Also - If you have the room, make the pad extend beyond the bins so any extra material that falls of of trucks can be tucked back into the proper bin.