Keep in mind that going the extra mile includes extra services you will offer along with your paid services – a translation project may not be over once you deliver the file, instead, they will probably get in touch to ask you to please check this list of inconsistencies, or check if this last-minute glossary is consistent with your translation. But let’s be reasonable – if you have a 5-page list of inconsistencies or a glossary with 100 items to be checked, it will take more than 10 minutes.
When a dear friend asks you a favor, and it is within your reach, why not do it? That doesn’t mean everything is asked for is within reach. You may be busy and not be able to stop for 5 minutes. Or that friend might not be really a friend and only get in touch when they need a favor.
There is no obligation to do more than requested for a client, and this should be done thinking about later compensation – which can be more projects (because you did an excellent job, so they trust you with more content), a yearly bonus, a big-fat thank you.
Going extra has to mean win-win. Otherwise, why would you?