I dislike combat fishing. My preference would be to not have anyone else in sight, but that's usually not realistic. If there are miles of good water that nobody's fishing, don't set up right next to me please.

I agree with Byron that 50 yards is usually fine.

Swinging for Steelhead is a different situation. The routine is to cast, swing , then take a step or two downstream and repeat, working your way through a run. In this case it's considered rude to step in below someone, on the same run, even if your 50 yards downstream. It is usually OK to start upstream and follow someone through (but polite to ask first).