[williamhj]

If you do some searching you'll find other threads discussing this is great detail, sometimes with great passion.

I don't think about it too much and use both. The one time I prefer a down-eye over a straight eye is when tying parachute patterns, since it gives me a touch more room between the hackle and the hook eye. But I'll happily tie them on straight eyes if it's what I have around. I like the look of down-eyes better personally, but don't think it matters fishing.