A/C Charge by Myself

A) A 30 lb can of refrigerant has the tap on top and only lets out gas. In order to worry about getting liquid on the low side and slugging the compressor, you'd have to turn it upside down. B) The pressure coming out of the 30 lb'r is way less than the pressure you will see on the high side line, so it would just pump it back into the cylinder if you opened the valve. The low side will see around 25-35 psi, the pressure out of the cylinder is somewhere around 70 psi depending on outside temp, so by hooking it up to the low side, it sucks it in and is forced in by the pressure differential. But to each their own.