I posed a question to a certain Baba (holy man) in India regarding the use of cannabis in the morning before doing yoga. He told me of charas (Hashish): "Charas is Shiv Prasad (a gift from Lord Shiva). It has the power to free your mind but unless you be of strong will and give full respect to this sacred preparation, your body will surely fall into the pit of sloth."
The practice of yoga while high on pot is not necessarily contrary to yogic philosophy...as long as we actually practice. According to Kalidas Baba, few and far between are the Babas who "wake and bake" while still managing the consistent practice of morning yoga. He maintains also that in the ritual smoking of the chillum (traditional clay tube used for smoking charas) during daylight hours, assana (posture) is of the utmost importance. To lay down, recline or give in to the urge to over-eat (munch) is to surrender to the tamasic aspects of ganja and the fall to slothfulness will certainly follow.
Finally, here's a quote from another famous 'Yogi' which I find quite fitting.
"In theory there is no difference between theory and practice- in practice there is." -Lawrence Peter "Yogi" Berra, American Baseball Legend (born May 12, 1925)