Wiggle Room on the Toss

Hi guys. I thought the rules stated the toss has to be straight, but I notice a lot of accomplished players toss  a parabola where the ball comes back toward them. It's parallel to where they release it, but definitely not straight up and down. They're certainly not hiding the ball, but is this legal?  

Hi Thomas,

This is often a point of contention.

This is what the guidelines are but I find the implementation is difficult or not applied very often.

The server is required to throw the ball “near vertically” upwards and it must rise at least 16 cm after leaving his or her hand. This means it must rise within a few degrees of the vertical, rather than within the angle of 45 that was formerly specified, and that it must rise far enough for the umpire to be sure that it is thrown upwards and not sideways or diagonally.