This jewelry tutorial video shows a trick of how to drill an accurate straight hole in metal. The more accurate your drilling the more neat the eventual gemstone setting will be.
With this drilling method it is important to drill short burst using your rotary hand piece and then rotating your ring clamp or the metal by a quarter ( 90 degrees). You continue short drills, turn the metal and drill again. This will correct any tendency of a drill wondering to the one side resulting in a skew hole.
This prong setting tutorials is a good example where accurately drilled holes are essential.
And this halo ring making tutorial is all about accurate drilling.
Click on the picture.