it depends on what surface you're using. those optical sensors have a hard time "seeing" if the surface is very white and shiny or if it's clear, like glass. i have a glass computer desk and i need a mouse pad for my mighty mouse.
but, it need not be a mouse pad...mouse pads were originally texturized for those rolling ball mice but you can use a sheet of dark paper, for example, which would work perfectly. just continue using your mouse pad...there's no "right" or "wrong" by using a mouse pad.