Can I fly solo when practicing IFR under the hood?
Before going into the FAR AIM, take a second to think about this idea:

You are in your airplane, under the hood, flying along your route or wherever. The only traffic warnings you will get are from your controller, and he does not always see all the VFR traffic on his screen... Not a good idea. Definitely never fly without a safety pilot under the hood in VMC.

Now lets open the FAR AIM:


  §91.109 Flight instruction; Simulated instrument flight and certain
  flight tests
  
  (c) No person my operate a civil aircraft in simulated instrument
  flight unless - 
  
  (1) The other control seat is occupied by a safety
  pilot who possesses at least a private pilot certificate with category
  and class ratings appropriate to the aircraft being flown.
  
  (2) The safety pilot has adequate vision forward and to each side of
  the aircraft, or a competent observer in the aircraft adequately
  supplements the vision of the safety pilot; and
  
  (3) Except in the case of lighter-than-air aircraft, that aircraft is
  equipped with fully functioning dual controls. However, simulated
  instrument flight may be conducted in a single-engine airplane,
  equipped with a single, functioning, throwover control wheel, in place
  of fixed, dual controls of the elevator and ailerons, when—
  
  (i) The safety pilot has determined that the flight can be conducted
  safely; and
  
  (ii) The person manipulating the controls has at least a private pilot
  certificate with appropriate category and class ratings.


Getting the important stuff out of there: No person may operate a civil aircraft in simulated instrument flight unless the other control seat is occupied by a safety pilot!

The FAR/AIM part obviously only goes for FAA regions, but I'm assuming that is what you are asking about.

Conclusion: According to logic thinking you should never ever not look out the window in VMC, and the FAA doesn't allow you to either, not even for IFR recency!