If refrigerant needed to be added last winter and the leak was not repaired then you will probably be low on refrigerant again. A good service person would have found the leak in your system and repaired it. By just adding refrigerant with no leak repair it will leak out again and again until the leak is repaired. This could get very expensive over time. You may try looking around the system for oil deposits since oil is one of the most common indicators of a refrigerant leak.