I disagree with some of you--not everyone does it. It's cheating. It's like making the net lower, the player taller, the service box bigger. I ran some trig on it once, and every inch a player steals really effects the geometry of the court. Also, don't forget the ball is traveling faster.
I usually wait for an ace and then say something like "nice serve--think you can hit that one from behind the line?". Or even better, if they question a line call on their serve, you could say "you had the length right--you just stood too close."