I couldn't locate anything that says a motorcycle can be/should be backed into a curb spot. Only the parallel parking description below is on the D.C. DMV location.
Test Question97. A motorcycle may be parked:
a. At a right angle to the curb between cars
b. In any vacant space provided the motorcycle
is parked parallel to the curb
c. In any parking space provided for a fourwheel
vehicle
97. c
2400 PROPER PARKING: GENERAL REQUIREMENTS AND
PROHIBITIONS2400.1 No person shall stand or park a vehicle in a roadway other than parallel with the edge of the roadway headed in the direction of lawful traffic movement.
The Virginia State Motorcycle Guide actually state to park at an angle:
At the roadside-Angle your motorcycle so that you can see in both directions without straining and without having any part of the cycle in the lane of travel. Angling your motorcycle so that you can get a clear view in both directions is particularly important if you plan to turn across a lane of traffic.
http://dmvnow.com/webdoc/citizen/drivers/mcmanual/mcmanual6d.asp