No matter what kind of serve you hit, the contact point is always on the BACK of the ball -- right smack dab in the center of the "clock face."
Many players are confused by the clock analogy. They think it means that you roll the racket head around or over the ball in serving. But you don't. The moment of contact is too brief. Your racket doesn't stay on the ball for more than a few milliseconds. You get spin because of the direction your racket head is moving at contact. The strings bite into the fuzz on the back of the ball and brush that spot just a teeny-weeny bit upward and/or rightward while hitting through the ball. That's what imparts the spin.
If you've studied physics, this will sound familiar: Picture your swing in three dimensions. Or slowly go through the motions of a swing before a mirror. You're not just swinging forward, are you? Your racket-head is also tracking upward and/or rightward a little at the point of contact. These upward and rightward
vector components of motion create spin. The
forward vector component is the main one. It drives the ball forward.
So, the clock-face numbers have nothing to do with where you contact the ball. They just indicate the direction your racket head is tracking at contact. In other words, they indicate the direction your racket should brush the back of the ball = the direction of the spin.
When hitting a slice serve, it may
feel like you're striking the right side of the ball a glancing blow. (It does for me.) But if you really do hit the right side of the ball it won't go over the net -- it will go LEFT, into the next court.
Pure slice spins sideways, from 9 o'clock to 3 o'clock. In theory. In practice, heavy slice only comes close to this.
Spinning from 8 o'clock to 2 o'clock gives you mostly slice with a little topspin. Spinning from 7 o'clock to 1 o'clock gives you a little more topspin and less slice. And so on.
Pore topspin would spin from 6 o'clock to 12 o'clock. Again -- in theory only, because you can really only come close to that.
For more slice toss farther out to the right. This way your swing tracks sharply rightward. For more topspin, toss more overhead. This makes your swing track sharply upward.
Don't worry about trying to hit all kinds of serves from the same toss. That's one of those apocryphal bits of conventional wisdom that just doesn't hold true. Even the pros vary their toss to hit different kinds of spin, whether they admit it or not.
After you're mastered spin, you can try to toss to a spot midway between the best place for slice and the best place for topspin, trying to hit both types of spin from that toss. But you won't get as much slice or topspin that way. And the gymnastics you must go through to get the proper angle of attack on the ball are as much of a giveway as varying the toss. In fact, that awkward motion is far more noticeable than just varying the placement of your toss a little.
Besides, the only time the receiver can benefit from reading your toss is if it's way too high. Then he has time to adjust while you wait for it to come down to where you can hit it. But the the problem isn't varying your toss for different spins -- the problem is tossing way too high. That hurts you in many ways.
There's a whole series of pages on spinning serves starting here:
http://www.operationdoubles.com/spinningserves.htm