I agree with Madriverfisher - too many factors. Depths, water flow/speed, light, width of stream/river, hatch, ect... My own personal habit is to enter at a certain spot I think is good and work that section both ways - up and down. From there, I'll usually try to wade upstream and cast upstream, but not always. If I'm working downstream for whatever reason, I'll wade carefully usually on the edge if possible to create the least amount of disturbance possible. Hunting skiddish trout in shallows sometimes requires a stealth approach, and any given target should be accessed from the best direction, whether upstream or down.