I think that it is best to try to keep low while hitting the ball, but can see where this could be problematic for some.  The goal of hitting the ball low would be to keep the plane of the racquet paralell to the floor.  Thus resulting in shots that stay on that paralell  plane.  In this way you can reduce the likely hood of missed shots that are high or low of the target and result in shots that are left or right of the desired target (More desired result).  As for during the rally, I would say that being in a ready position is good , but that being low is not necessary.  If you watch Sudsie, Kane and some of the other pro's they often stand straight up walking in to position, awaiting their opponents shots.

I would say another tip especially for beginner or intermediate players is to keep your eye on the ball.  lsounds cliche, but I see it all the time.  It seems that people fear looking back to watch a player shoot when they are in the front court.  Possibly concerned with getting hit by the ball etc.  Watching the ball is a vital part of the game and rreduces the likelyhood of getting hit by a ball or worse a racquet.  This is especially true for safety when playing doubles.  Watching the ball allows you to anticipate the other players next shot, helps you to decide the best position to be in on the court and also reduces the likelyhood of getting hit by a ball or racquet.  For example:
If you hit a shot in to the back court and then stand facing the front wall, you have no idea where the ball is going, when the other player is planning to hit the ball and with your opponent following the ball off the back wall etc. they are not able to keep an eye on the other player to ensure that they are also in a safe position. 

One thing that I do and would recommend to others is to put the racquet up to your face and watch the ball through the strings.  In this way you are able to keep an eye on the ball, move to a safe and ideal spot on the court to set up for your next shot and it allows you to anticipate the other players shot and get a jump on your shot.  The racquet will help to ensure you do not get hit in the face and allows you the ability to get out of the way.  

This is a huge part of court awareness and with the speed of the game at the higher levels, will help you to anticipate the and  improve your game.   
BP