In most bars, should a customer offer to buy the bartender a drink they Will usually take a cup and write on it so they can have it at another time.
The bars that I worked at would allow us 1 free shift drink at about 1:00am before the end of the night for those who closed the bar.
others were allowed to have a shift drink after their shift.
You do have those staff that will hide and sneak shots all night, they normally do not last long at employment.
If you didn't do your shift work cause your hammered, then you will end up looking for a new job.