Fluorescent lights are omnidirectional, meaning they put out light in all directions. My thoughts would be to reflect as much light back down (assuming whichever way they are mounted they are over the plants...lol) on the canopy so you don't waste whatever side of the light is facing away. Aside from that if there is a restriction for mounting them that is based on performance, it should be on the package from the manufacturer.

To my knowledge, there is no study that available that will illustrate the differences in mounting angles of CFL bulbs, but I would love to be wrong and see one.