I think the most important thing with a dry fly is size and imitating the way the fly interacts with the surface of the water. With nymphs the most important thing is a "general impression". I think too much realism in a nymph is often counterproductive because all of those appendages and antennae hanging off of your fly aren't acting the way a fish would expect them to.

Here's a little aside that sort of applies. When you pick up a scud what does it look like? It curls up in your hand right? Have you ever seen one that is swimming? They are board straight when they swim. The next time you're tying up some scuds try tying them on a straight hook like a TMC3769. I think you'll be pleased with the results on the stream. A scud tied on curved hook looks cool. A scud tied on a straight hook looks a little silly. I think you'll be surprised at which one works better, though.

Having said all of that, many time fly tying is about what you can create. It's just as much fun to tie something that pleases your eye as it is to tie something just to catch fish. Fortunately, especially with dry flies, they can often do both.

Jeremy