If ASA, Supplement 40-B (pages 131-132) states : "Signal. A pitcher must take the signal or simulate taking the signal from the catcher while in contact with the pitcher's plate as described in (A) above. The ball must be held in the hand or glove/mitt while taking the signal. The ball held in one hand may be in front of or behind the body. Taking or simulating a signal prevents the pitcher from walking onto the pitcher's plate and putting the batter at a disadvantage by throwing a quick pitch. The actual signal may be taken from the catcher, the dugout or any other location."
So, as long as they simulate a signal, the actual signal can come from anywhere. Again, the above is per ASA only. Hope that helps.
I have a funny book called Little League Confidential, where the coach just yells out all of the "signals." Like "steal!" "bunt" "change up", etc. He says that the other coaches really get mad, because even if their team knows the play that is coming, they still don't execute against it.