Even if the advantage gained by not varying the toss is small, why not capture it? As with any other gain, the key question is the cost: how easy is it to hit different spins with the same toss? Proponents of the same-toss theory often cite Pete Sampras as proof that it can be done--and extremely well. If you watch video of Pete's serve, you will see that he does almost always toss to his left, and he does hit those tosses with spins ranging from a 50-50 topspin-slice to a full twist. By watching closely, though, you'll see that the toss for his topspin-slice serve, his usual first serve, is typically around two feet to the left of his ground centerline, a vertical line rising from halfway between his feet, but the toss for his twist serve is more than four feet to the left of that centerline. Pete manages to hit a topspin-slice on a ball two feet to his left by leaning even farther to the left, thereby putting the ball to the right of his head, where it needs to be to hit that spin. He leans even more to the left for his twist serve, so much so that it looks as if he might fall over, but his toss is so far left, the ball remains slightly on his left, where it needs to be to hit a strong twist serve. It simply isn't true that Pete Sampras uses the same toss to hit different spins; he almost always tosses to his left, but his toss direction varies by at least two feet, as much distance as if he tossed between a foot to his right and a foot to his left.
To hit a topspin-slice (commonly called slice) serve, you have to brush up from the center of the ball toward 1:30 if the ball were a clock face. If you try to do this with the ball to the left of your head at the moment of contact, your head will get in the way of your shoulder, and your serve will fly much too far to your left; thus, the only way to hit topspin-slice with a toss to your left is to lean, like Pete, farther to the left so that the ball is on your right.
To hit a twist serve, you have to brush up from 7:00 on the ball's clock face toward 1:00. If you try to do this with the ball to the right of your head at the moment of contact, your serve will fly much too far to the right, and you won't get enough topspin. This problem can't be solved by leaning to the right, though, because to hit a twist, your whole upper body needs to be rising along with your arm from left to right; you have to start a twist by leaning to the left.
Hitting the other full kick serve, a pure topspin, wouldn't work either with the ball to your right, because your arm isn't long enough to swing a racquet straight up fully on a serve and still reach to your right.
If you want to try to hit a decent range of different spins with the same toss, therefore, you have to toss every ball to your left, because tossing to the right eliminates the twist and pure topspin. You could probably manage to hit a range of spins from twist to topspin-slice by tossing every ball around a foot to the left of your head and then leaning left by varying degrees to accommodate each spin, but you couldn't hit any serves with more than around 50% slice, the sidespin component of your twist might be limited, and you would meet your topspin-slice somewhat lower because of your lean. Whether these costs are worth the potential benefit described earlier is up to you.
Let's see what several of the best servers in the pros do.