When flyfishing, I generally abide by flyfishing etiquette of starting at the top of the hole and working it to the tailout, except at the terminal fisheries (Reiter, Fortson, etc.). But when working gear, I don't think it's a sin to step in downstream of another angler. Not crowding in, mind you, but if you have a 100-200 yd run with one person fishing, I don't think you should have to step in upstream of him.

I guess the reason I think this way is because with the fly rod, your generally limited to a 50-80 ft swing and the fish that are in this radius. There is also less variety of what you can throw (compared to gear), so if your working the water first, you should be able to present your stuff to the fish first. You're also more likely to wade further out and if someone steps in downstream of you doing the same, disturbing the fish, you're out of luck. With fly gear, you're also more apt to get the aggressive ones and putting your presentation in front of the fish first, is more important.

But with gear, the water you can cover is so much larger, and you can use bait, change bait, not use bait, swing with drift gear, free spool, long line with floats, throw hardware, etc. I also will "camp" on a spot for a reasonable time if I think there are fish, because with the variety of gear that can be thrown, you can get shy fish to pick up eventually. Anyone with any amount of time spent on the rivers will tell you of the person that hit a fish standing on the rock you just left; gave them something different. I've done it and had it done to me. With so much more water that can be covered with a variety of gear, I just don't see a problem with stepping in downstream of another gear angler, as long as I'm not crowding them.

If the run is very narrow such that everyone is fishing the same water, then I would probably ask to step in or just step in at the top.