I would think it would be best to watch anyone already fishing before assuming they are fishing one direction or another.
Three weeks ago I was fishing one of my favorite roadside Little River spots with a dry and a nymph dropper. I had just rigged up and began to fish with the intention of actually fishing downstream or with the current. No sooner did I begin to fish and move that way only to have another car pull up, the driver and his son get out and move about 20 to 30 yards ahead of me in the direction I was heading. I have to admit I was tempted to be quite angry, but tried to extend mercy and just didn't say anything. I later spoke with the fellow and discovered he evidently knew enough about fishing that he should've known and done better so at this point I figured it would be useless to complain.
Now as a result of this interruption, I decided to fish upstream and caught a nice, nearly foot long Brown. On top of that as I released the Brown that I caught, an Otter appeared to afford me a bit of impromptu entertainment. All in all things worked out, but it would be nice if folks asked before stepping in or at least observed a few minutes before they made an assumption and irritated or frustrated other folks.
"Fly-fishing has many attributes, but none more pleasing than it's ability to liberate the young boy that still hides within me and to let that boy live again without embarrassment or regret, sorrow or anguish." Harry Middleton