No, it is not a common practice since there should be a dedicated "fill port" fitting included in the valve end of the cylinder.
But I don't know where you have been getting your tanks filled.
There would not be any harm done to the tank if a fuel vendor filled the tank through what is the delivery valve, but I can't see any reason why they would do that as it would take a bit more time to fill it through the smaller delivery port than the much larger fill port.
The tank should really be filled by weight using scales.
In lieu of weight filling, the tank should only be filled until a small shot of liquid propane escapes from the very small purge valve while the tank is oriented in the horizontal position with the tank index pin slot/hole at the 6 o'clock position (same as when the tank is in its horizontal mounted position).
These tanks must never be filled more than their specified volume by weight or above the level of the purge valve at horizontal. This is to assure adequate room for thermal expansion of fuel and pressure increase from same.