Just a quick story about this very scenario . . .
I was watching a men's college basketball game this past Thanksgiving on South Padre Island. Ball "appears" to be deflected by the defensive player into the backcourt. Trail gives the "ball tipped" signal indicating that there would not be a backcourt violation. Apparently the Center did not see his signal but did have a bead on the BC violation because he (the "C") blew his whistle and ran over to signal a backcourt violation. Made the Trail look really bad, especially when they began to debate the call . . . From my vantage point in the front row right in front of the play, I think the "C" should have been watching the his half of the court instead of watching the ball.
I agree with many of the other posters in that there are some situations when you may want to use this "ad lib" signal, however, as one poster said, be certain to cover this in your pregame so as to avoid miscommunication. Personally, I don't use the signal because I don't want to have a situation like the one above.