If you top it off, and it continues to cool and operate normally, then I'm sure the leak is VERY small, so I guess I would weigh the cost of finding and fixing the leak vs. the cost of topping off the R12 and compressor oil periodically. The leak WILL grow over time...it's just a matter of how quickly.
There is also this issue: It is "technically" illegal to add R12 to a system that is known to have a leak. Most shops would not do it for you. I think the whole R12 thing is bulls**t anyway, but just in case you weren't aware....
Mike