Fans seem to fine the way you have it set up. Setting a PC on the floor wont hurt it, but it will gather dust vary easy tho, Just walking by it and pets scratching by it or playing in front of it will push the dust and hair in the air and into the PC fans, so more cleaning really. But it wont hurt it at all. You can use the floor tile to make sure the air does flow under the case for like the powersupply or intake fan at the bottom if you have 1.
I have an hx750 which which is bottom mounted, I am not sure whether than fan is an intake or outtake, do you know?