Your reasoning is "if the police are coming..."
That's an awfully big "if"
If I heard sirens, I agree I'd hold off. Same if I noticed uniformed people sneaking toward the doors.
Short of that, I don't know that I'd assume the police are on their way. Unless the clerk has an alarm button he/she can press, and has been in a position where they could do so, or unless some unnoticed witness has called the robbery in, then why would anybody in the store assume that the police are on their way?
It's not like the police stop by CVS every few minutes to say hi.
Now, if you were in a position where you could stay fairly well hidden, monitor the situation, and call 911, that's something else.