If you check out the thread "Which fly would you choose", you'll find the answer isn't very conclusive! Although it does seem that the region you fish in, the type of water you're fishing and the size of the trout that live there have a pretty strong bearing on the choices! You could start by reading some of the answers in that section.
What area are you from and what waters would you be fishing?
I am going to say, it depends on Stillwater or River, but I will use nymphs on both, I rarely us streamers on rivers (I know I should, but I usually use bamboo, so I don't)
Lakes however, streamers are a must!
When guiding clients here on the Snake I use dries about 85%- 90% of the time. When fishing myself I use streamers unless there is a good hatch going on, then I'll switch rods and tactics. When float fishing I always have my 5wt ready to go with dries. I use my 6wt for streamers.
When I lived on the Main Line outside Philly I fished mainly dries and nymphs. Size of the water and the fish are factors to consider. As well as insect activity.
Haven't you ever spent a day on the South Fork chuckin Streamers? It's my favorite!! Got box full of big juicy ones. Call it my Trout Candy Box. LOL. Thats the way I got that browny in profile pic. Caught him just above Bighington.
ill be fishing the pere marquette, and the au sable rivers in michigan, au sable in the spring, and pere marquette late september and early october i dont know if this matters at all but we fish for salmon usually on the PM with spinning tackle, but im planning on taking the fly rod up there this year, will the salmon scare away the trout?