@RalphH: "In my experience it is pretty much unnecessary once you coordinate hook size, drift angle & retrieve speed."
Could you explain that a little more? Thanks.
Basically you don't want your hook dragging down among crowds of fish. You want it above the fish though not too far above or they may not move to it. Coho and pinks are more inclined to move a good distance to take a fly or lure than chum or springs. You also want the fish to see the fly before any other part of you presentation and preferably not see anything else. Generally a slow moving fly is better than a fast one.
With a fly I have experienced using the same sink tip and fly going from multiple fouling hook situations to nothing but fair takes simply by adjusting the angle on the swing of the line. That was all it took to meet the conditions above.
Size of the hook, if it's got a bead on it, the length of the leader, the size of the fly line, the density of the sink tip, speed of the current and or the retrieve all effect where the fly will be in the water column and how fast it moves.
Over the years I have almost exclusively switched to using a clear intermediate line or less frequently a floating line rather than a faster sinking type 3 or denser tip. That alone cut foul hooked fish as a ratio of hookups to a fraction of what I experienced previously.
Overall it's just a matter playing or adjusting the parameters that effect the depth and presentation of the fly to all but eliminate snagged fish.