11-202v3 AETC Sup: 
  
 
8.18. IFR "VFR-on-Top. "  Unless otherwise restricted by the MAJCOM, PICs may request  
and fly IFR "VFR on Top" operations (IAW AFMAN 11-217V2 guidance) when the mission  
requires such clearances.  If the PIC cancels the IFR clearance, comply with Chapter 7. 
  
8.18. (AETC) IFR ―VFR on Top. ― VFR on top operations are prohibited unless prescribed  
in AFI 11-2MDS-specific or syllabus guidance. 
  
Also: 
  
 
7.3. Flight Operations under VFR. <snip>  PICs of VFR aircraft must at all  
times be able to control the aircraft by visual reference to a discernible horizon, regardless of  
cloud clearance requirements. 
  
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It is specifically prohibited by AETC.  Other MAJCOMs can make their own rules.  Personally, I'm not a fan of the VFR on top plan of action because of 7.3.  Sloping cloud decks often give a false horizon and if you've every flown while looking at one, it's very disorienting.  Also, the plane I fly hardly ever is below FL180 anyway so it's a not really an option anyway.