The closer to the limbs you place them.. the less speed you'll lose. The closer to the nock you move them.. the quieter the bow will be.. within reason.

The leeches should be served into place... and should not move on you.

I personally use rubber cat whiskers placed at the very ends of my string a few inches from where the string meets the cams. I have these trimmed to the size of a golf ball. My bow was very quiet.. and I didn't lose any speed.. so I got the best of both worlds on that one.