From the Hook&Hackle website.
Q. Do I have to put the guides on the belly of the rod or can I mount them on the spine?
A.This is a question often discussed by rod builders. Some feel very strongly about their method of guide placement. One thing is for sure the guides must be placed on the spine or the belly or else the finished rod will exhibit "sideways" torque. The spine results from a number of vectors in the rod blank. If the guides are placed on one side or the other of the spine, the vectors will be front to back or vice versa. If they guides are offset, the resulting vectors will be too. During casting one feels the result as the casting hand tends to turn in or out during the back and forth motion. This gives rise to a number of casting problems. One school of thought suggests that lighter weight rods have the guides placed on the belly (2 thru 6 weight lines) and heavier weight rods (7-13) have the guides placed on the spine for more efficient casting or retrieval of that particular line weight.
The simpler the outfit, the more skill it takes to manage it, and the more pleasure one gets in his achievements.” --- Horace Kephart