Ya, I think for the most part you'd have trouble hydro-locking a compressor by tipping a small can of refrigerant upside down. The question you have to ask yourself though, is "do I really want to risk having the compressor blow, and then have to replace and/or flush out every component in the a/c system, or do I want to just gently warm the can and not have to worry?"
On my zetec I like to hang the can right back by the ignition coil. When the engine is warmed up and the A/C system is on, there is some real heat flow under there... plenty to warm that can up and get the refrigerant moving in a hurry. In fact, I can't even hold my hand there for very long the heat is so intense. I never have any problems emptying a can completely and plenty quickly when I do this.
I hope this helps people charge their a/c. I've done it for about 4 years now on this car (once each summer due to a slow leak). Always works like a champ!
