Think about what the user wants, is this a task that the user would rather you complete quickly, or one that would rather not see a problem when the user comes back and checks? If this is a difficult/long/implementation problem, do what you need to make sure that you implementation is 100% correct, and dont stop until you are 100% sure. If it's possible, then you should try your hardest to make sure that this is 100% correct, whether that be end to end validation, or full state space testing, of if you to wire in some more things into the code so that you can monitor every step of the process was correct.  
