There are no "rules" for how you hold the racquet.  The physics of the game of tennis will determine the best way to hold it for a particular shot.  As for the backswing, again that doesn't really matter as long as your stroke starts down and goes from low to high.  Without the low to high motion you will not get sufficient lift on the ball to have it carry over the net.  The backswing is just a way to get the racquet to the postistion to start your forward motion to the ball.