Good question. I've often wondered about this too.
My experience while living on the beach and fishing around B.Isl. has shown me the following:
1. The steelhead I've caught were always incidental. All were hungry downriver fish in early summer while using very light tackle (spoons, etc.). Always shallow, but along both shoreline and in open water, in the salt and fresh (Lk. Washington). Usually fishing for cutts or trout.
2. We used to watch them cruise along the shoreline and under our dock in very shallow water on their way to spawn in late fall and winter. Usually solitary, very spooky and making tracks.
Seems to me that steelhead really don't hang around the Sound much or they would show up in salt and Lake catches more often. I'd be surprised if they schooled like salmon.
Just some thoughts and obs.