Over a long period of time it is better to have the primer button facing up or horizontally. The reason is that as you run propane and it is heated through the vaporizer there will be "heavy ends" that drop out of the vapor and build up. If you can keep your water temperature at the vaporizer below 180 degrees it is not a problem. But that is sometimes difficult to do. If the "heavy ends" drop out and the secondary diaphragm is below the housing then the residue will attach itself to the diaphragm and change the weight of it and response. Also landing(bouncing) can affect the diaphragm and cause it to shut off the fuel if in a downward position.