In a professional working environment: Never. Except it is in your service-contract but then I wouldn't consider it as "backdoor". As you framed the question I see it more as a question aimed at morality. Everybody has to decide that on their own. From a legal side of view: Don't do it. That backdoor that opens the code also opens many doors for lawsuits (from all sides). And always remember: There is always someone that knows more than you ;)