At our venue a 20% gratuity is already added to our food cost. They however do not have a gratuity added on top of the open bar costs. Open bar for us is paid on a per drink basis and we have a ceiling limit (we expect our ceiling limit to cover a majority of the reception). Do we tip the bartenders at the end of the night or do guests as they order? If us, what percentage should we budget for gratuity?