Depends on the style of rack you choose.
The owner's manual states that you only need 1/2" clearance above the fan opening. I put my 1001RB-II in a two space SKB X-Rack (the short style with the trap door in the back). The way the molding of the case is at the top, there ends up being plenty of room for airflow with this design, even with this amp in the topmost position, so I don't think with this you'd need to budget for a whole extra rack space.
I wouldn't do this with a regular flat panel style of rack, however (the usual Anvil or Calzone style construction), although I've never eyeballed it to see if there's a half inch of clearance at the top or not (there could be).
But I've never a a problem with my two space X-Rack.