Our floor is level, with a floorgrate as you describe. Aside from a little extra work, it has been fine for us. Most anything tends to run towards the drain as its really the only place to go anyways. Anything that escapes said vortex gets pushed in with the water hose...it is a tad less work that the squeegee
We do have a problem with spent grain clogging the drain screen, but that's just when we're sloppy emptying the grains and too much gets on the floor. The addition of a slope isn't going to fix that though.