Re: Hook Eyes Up/Down/Straight?
Keep this in mind, whichever style of hook eye is going to (theoretically) slightly affect the way the hook point is set and pulled against the fish. This has to do with the angle at which the line is pulled in relation to the hook eye (which is again in relation to the rest of the fly). To put this visually, if I stuck a hook tip in a piece of wood and pulled the line attached to the eye at a completely parallel angle to the hook shank, some different things would happen depending on the eye position.
If the hook eye was up: The tension on the line would attempt to pull the eye down to the level of the shank (which is the level you are pulling the line at). This slight level adjustment would affect the entire angle of the fly. As the hook eye goes down, the hook back of the fly points upwards slightly, driving the downward facing hook point further down. This (in theory) would actually seem to increase hook set potential. I say this because if the hook point is gliding along a level surface, forcing the point slightly downward drives it into the surface more.
If the hook eye was down: An opposite effect would happen. Pulling the line at the same level as the shook shank would drive the head of the fly up, in return, driving the back of the fly down. All of this in turn drives the hook point up, creating a situation that makes sticking the harder given that the point is sliding along a level surface.
If the hook eye was center: Pulling the line would drag the hook point at a steady level along the surface, giving it an average sticking potential.
I know this probably way more in detail than most people see it and maybe this isn't actually realistic behavior, but it is interesting.