TradeShowDave:

Here is the OSHA position on filling propane cylinders by volume.

Part Number: 1910
Part Title: Occupational Safety and Health Standards
Subpart: H
Subpart Title: Hazardous Materials
Standard Number: 1910.110
Title: Storage and handling of liquefied petroleum gases.

1910.110(e)(7)(iv)

Portable containers equipped for volumetric filling may be filled in either the vertical or horizontal position only when oriented to place the safety relief valve in communication with the vapor space.