You might think to yourself, why would anyone care about the warmup? But it's important from both a mental and physical point of view.
The warmup is not the place to try and fix problems with your strokes. Its purpose is to get your muscles loose and your blood flowing so that you won't injure yourself during the game. Once you reach that stage, you should be ready to begin -- after a total time of no more than 5 minutes.
If you're in good shape and not worried about getting tired, you can do a bit of light jogging or lift some light weights before going out on the court. Then while your opponent is loosening up, you really can try to make sure your strokes are all working OK.
Some people rally for a long time, then take one or two practice serves and say they're ready. Or recreational players might even use the "first one in" convention. Serving is one of the most common ways to injure yourself -- wrist, elbow, shoulder, stomach -- so don't skimp! You should start with slow second serves, and work your way up to harder serves.
There are a number of reasons why you shouldn't bash the heck out of the ball during the warmup:
If your opponent gets carried away with hitting winners in the warmup, what can you do? Don't bother to scramble from side to side, just let the ball go if it's out of your reach. Conserve your energy for when it really matters, move around enough to loosen up your legs, and don't let your opponent know just how fast (or slow) you really are. If they hit passing shots when you're trying to volley, calmly go back and get the balls and go back to the net until you've had a reasonable number of chances to volley.
As you are only going through the service motion to loosen up your arm, it doesn't matter if the ball goes out by 1 foot or 5 feet. But if you serve into the net, that means more retrieving and stooping which just wastes everyone's time. By the end of the warmup you might try a couple of net-skimming ace-speed serves, but things will go smoother if you basically just aim well over the net.