Topping off an R12 system being illegal is a MYTH! In the material studied and tested on for a 609 certificate, it makes this very clear. In fact it is ILLEGAL for a shop to tell you this. They are either ignorant of the law, or they are trying to soak you for more repair expense.
That said, it is certainly not WISE to top off any system if there is a significant leak.
If you need verification of what I am saying you can go to any of the sites where you test for 609 and read the material.
Have a great day,