Not so much. Under both BCAPL and WSR, any part of the tip may be used, including the "front" or "top", depending on how you want to describe it. The test for a foul is a "forward stroke motion" of the cue. However, I don't recommend the practice. BCAPL 1-38-1 and WSR 1.5 second sentence apply.
B
This always worries me enough that I'll tell my opponent, (if I choose to adjust the position of the CB with my stick) "I'm adjusting", kind of like in chess. And make sure he acknowledges it. Generally, I won't adjust with the stick unless I'm already stretched out on the shot, and don't want to go around to move the CB again. Too easy to have an "oops" with the stick, and then hand the CB back.