With the can upright, you will introduce vapor into the system.
Upside down, you introduce liquid.

Which way you hold the can depends upon ambient temperature, and distance from the low side charging port to the compressor.

The higher the temp, and the farther the port from the suction side of the compressor, the lower the risk for intrducing a slug of liquid into the compressor.

Now, we all know what happens when you try to compress a substance that will not compress, right?

If you are just using 12oz cans, and charging into the W124 low side port near the forward firewall on the driver's side, then you should have sufficient distance for the relatively small amount (volume) of liquid introduced into the low side to evaporate before being compresed.

When the low side is located close to the compressor, the risk is high, so you'd want to avoid the upside down can trick if the low side port is within a couple of inches of the compressor.

Another trick to get optimum cooling is to watch the low pressure return line temperature. When you have the proper amount of freon in the system, this return line will start to become very cold.

You can also speed the rate of gas injected into the system by warming up the R12 can. Set it on the radiator hose or other hot engine area. Just be carefull to check the can when picking it up. It'll get quite hot once the freon has left the can......

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles