Three points
1) It is illegal to vent 134a. Under section 608 of the Clean Air Act, it has been illegal since November 15, 1995, to knowingly vent substitutes for CFC and HCFC refrigerants during the maintenance, service, repair and disposal of air-conditioning and refrigeration equipment. R-134A is substitute for CFC-12 (commonly know as R-12).
2) In terms of oil lost, you probably won't loose much. You're much more likely to over oil, than under oil. It is also a lot easier to add oil, than remove it. If the compressor is quiet, then you have enough oil.
3) If you really want your system set up correctly, have the entire system evacuated and flushed. Then refill it with the correct amount of oil and freon. R-134A is not very forgiving. 1 oz too much or too little and the system will not operate optimally.