Depends on your preferance. I use the Syn custom S and personally I like them as low as they can get while still being able to whammy up a full 7 half steps without the strings buzzing.
Just mess around and see what you like. Worth noting that different actions are preferable for certain techniques as well.
For tapping it would help if you had a video of you doing it, may ne technique, action, tone set up, etx..