One thing I haven't mentioned yet is putting refrigerant oil in the system. You normally do this when recharging the system after replacing one of the major components. This is tricky. The service manual normally tells you how much refrigerant oil should be added with each major component replaced on the AC system. The thing is, most replacement components, such as compressors, often come with the prescribed amount of oil already in the part. If you add oil on top of this, you might saturate the system and it won't work as well.
In addition, it's not a good idea to mix R134a refrigerant oil with R12 oil. This becomes an issue if you're retrofitting an AC system. On top of that, it's difficult to add oil to the AC system with DIY tools. Professional AC equipment comes with provisions to add the correct amount of oil with each charge. Off-the-shelf cans sometimes come with refrigerant oil in them as well. My point here is that if you want to err on the side of caution, don't add any oil to the system. It's worse if you add too much and cause a problem. Look for refrigerant cans that have oil already in them. This will be your best bet. Otherwise, make sure you measure out the correct amount of refrigerant oil and ensure it is the correct type before you add it to the system.