ok first off, to properly service an A/C system it must be put into a full vacuum to remove any moisture in the system which will damage it, venting it to the atmosphere will almost guaranteed to introduce moisture from the air to the system.
second, to properly fill the system it must be fully recovered to remove all of the refrigerant, so the proper amount can be added back in.
third but most important IMHO, if your gonna be tossing in cans of r134a then DO NOT BUY anything that has a stop leak of any sort. it is a contaminant that destroys pumps, plugs up condensers and evaporators, and it does not even do what it says it will.... stop leaks. if any car comes to our dealer and is found to have a stop leak in it must be collected with a different machine to not contaminate and damage it, and be disposed of as a contaminate. Its hard to find r134a from the parts store without it but i would NEVER put that crap in my car.
lastly, your A/C will never work as good as a true service by a reputable shop if you throw a few cans in.
ohh and dont fall for the "synthetic" crap that claims to cool better, r134a is r134a. dont spend more money on something that will do nothing more for you.