I had a guy suggest that I put floor drains a foot or so away from the garage doors, and slope everything towards the doors.  This way when you wash down the floor you can start on the clean end of the garage and work your way towards the doors.  This also minimizes the amount of water that gets splattered on the walls.  The other reason he had was that when you have tables and chairs in the garage they are on the same plane and they don't rock around.  This also helps if you're a shadetree mechanic and use a creaper.  Because you aren't rolling around over the grates with those little wheels.  You also don't have a place for those little nuts and bolts to fall into if your taking something apart.