Just for the sake of completion, there are some bus services that offer real-time GPS tracking, so you'd know where the bus is and roughly how long until it arrives. Presumably no such service is available.

We go on to the age-old question: do you try to get the best average performance or the best worst-case performance? If it's the best average performance, go to the bus stop and wait. If you prefer to get the best worst-case performance, just walk every time.