As long as your system is blowing some cold and the compressor engages, you have positive pressure in the system and no chance of air/moisture/debris entering the system, hence, no need to evacuate, and you can simply top off the system as needed. Once the system has no pressure (the gage pressure is zero), then you have to treat your system as if it's contaminated.

And please don't use the 134 cans that contain stop-leak stuff. This will result in very expensive repairs later on.

__________________
95 E320 Cabriolet, 169K