What actions warrant a lifetime boycott of goods or services?

I have tried to maintain a personal boycott on German goods and services, since I don’t believe many German companies ever acknowledged or gave a complete accounting of their use of slave labor during WW2 or their participation in the design of extermination machines (particularly auto makers like BMW, VW, Porsche or Merceded-Benz). Mainly, this means not buying obviously German products. Can a 50 year old sin-atrocity be persistent? Moreover, now that the world is “flat”, and companies are multinational, does this even make sense?