It depends! If you are required to follow a particular Code or standard, you have no choice. The requirements for NDT in Codes and Standards are based on past experience and operating conditions to avoid injury or premature failure.
If you are not bound by any Code or Standard regarding NDT of welds, I would use common sense and select a minimum sampling (say 10%) of critical welds to verify weld quality. With this approach, you can always increase the number of welds for NDT.