How you weight a fly effects how it falls. There is no wrong way to weight a fly. Using a weight near the front will cause the fly to work very much like a jig used on a spinning rod. That is, it will fall hook eye first. This is a very good way to fish a wooly bugger in many cases so you really want to tie some this way. Placing the weight along a portion or the length of the hook shank will make the fly sink more evenly, more like a crappie jig. Tying the fly with the weight in the back will create a fly that will fall tail first. This is not the best way for a fly to fall (least natural) but there are times (limited in my experience) that this works. It does work well though when done with a crayfish imitation.

Also, tie a few of these up with bead and in front of the bead place a very small spinning blade. This works very well as well. I know there will be some people who will compair this to fishing 'Hardware' and not being a traditional fly, but spinner fly's were around long before most synthetic materials and most modern patterns so I disagree with that veiw.

Good luck and have fun...
Daren