I've always approached tying dry flies with the silhouette being the most important aspect, whether it is parachutes or Catskill style. Take an adams you tied with wings and one without; go outside and hold them up to the sky and look at each one from beneath, exactly like a trout would see it. You will notice a considerable difference between the two. On the fly with wings, they will stand out as a prominent feature clearly visable through the hackle. On the one without, it will seem almost as if the fly has half a body; unless, your hackle is too thick to see through, which would give it a very odd silhouette anyway. The larger the fly the more you will notice this.
I don't know if there is good science or anything to back this idea up, but thats what i have confidence in. However you tie'em, you won't catch many fish if you don't have confidence that the fly will work.
I look at all the features of the fly too. Tails, body size or length, ect....
-Kurt