Just like FormerTeller said, Always buy them all unless you don't have the funds. Then buy as many as you can. Even if there is nothing of value there (don't forget to check for errors!), simply take them off their hands and dump them elsewhere. It will help build that relationship so that next time, that teller may put aside the goodies just for you since you have done them the favor of taking them previously. You just never know what that little gesture of buying them will net you in the future.
-TwoYewts