I think as long as you have space around it for air flow in/out of the fans and enough room to comfortably plug-in and run your cables it should be fine.
If you are worried about dust get a case with filters and place your computer up off the floor, Mine actually sits on a shelf I specially made about 5 feet off the floor, somewhat behind the desk (it is a corner desk).
Keep a can of compressed air and just blow out the case once a month, more or less.
Hm... Thanks. However I'd like to ask why, I'm just curious.
That way you have access to the front of the computer (usb ports, disk drive, power buttons). Just logical thinking. That is how cases are made. Why would you face the front towards the wall?