Fishing upstream or downstream should really rely on one basic thing-- Presentation of the fly to their holding area!! Many holds cannot be reached by only presenting in one direction, due the various nature of stream beds and obstacles within. Theuse of various casts can be utilized to achieve the desired drift to the proper location. For example, although I dry fly fish working upstream most of the time (85%), there are going to be certain holding lies that i can only reach from a down stream angle (the seam/flat water right in front or between a series of boulders). Using a slack line stack cast you can feed out a dead drifted fly right into position working downstream. Another advantage of fishing a dry downstream in this manner is the fact that the first thing to reach the fish is the fly, so lining them is not an issue. The problem of course is having slack in the line. Spooking a fish by your location is also a concern, but you can do that from any spot on the water if you happen to be a clumsy wader that takes an occasional dunk like myself . My thought is that you can fish up or down with any style/method of fishing, merely dependent upon finding the best way to get your fly in front of the fish.......