Mark,
It is as everyone has said...
I will only look at the pattern hook size to see what size the person who created the fly uses and then I use the size I think will work for my area and style of fishing. I do think that you should look at the size hook requested by the pattern because if you do not, you might tie a fly that is suppose to be a midge and you tie it on a #8!!
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.