Well you can't have your cake, etc. If your system is low then you ether charge it or ignor it.
If you want to keep from charging it repeatedly then it would be smart to start the leak detection process at the same time. Find someone who does florescent dye testing and have them add the dye with the refrigerant. Don't make any decision about retro-fitting R134a untill you find your leaks. I wouldn't advise R134a in early 124 body cars such as yours; the compressors are under capacity at least till the 10p17c compressors came outin around 90. Even then I wouldn't put R134a in any system that you aren't changing the compressor as they will all leak in a couple years if not.