I am reading more posts about useless bartender at higher lvl..

so... try to calculate.. 

If you know, you will be 2 hours outside/offline, and you know the whisky gives the best profit, try to buy 20+ whiskys alongside with 30 wines and some beer...
it will be all sold with MUCH bigger profit than selling a beer non stop.

It's a part of your strategy how are you playing and using your opportunities