Im on a juice fast and none of my produce is organic, as a student, I simply cannot afford it (other than the odd item here or there) its a choice between organic, or not paying the rent!
Any juice is better than no juice at all, Im certainly feeling pretty good.
Just make sure that if its not organic, you wash everything thoroughly and remove the skins on standard skin-removing items (carrots etc).
Good luck juicing, its the best thing Ive ever done for my health!