I'm with Murray on this one -- I've been maintaining my right-hand nails for thirty years (as a classical guitar player who gradually extended the technique to other styles), and filing them straight across would not work for me. A gently rounded curve, extending all the way to the sides of the nail, is what I've had the most success with. This shape is strong, glides off the string smoothly, and allows you to change the timbre by slightly altering the angle of attack.