IMO, timekeeping is not a good way to determine whether the watch needs a service. If the lubrication has dried up, the watch may still keep decent time, but friction between moving parts will gradually damage the movement as it runs. Moreover, the timekeeping provides no information about other aspects of the movement, e.g. the auto-winding. If you plan to only use the watch on rare occasions, it may not be worth it to you to pay for a service. But if you plan to use it regularly, you might want to have it inspected by a watchmaker. If it was recently serviced, the watchmaker should be able to tell.
