When is it appropriate to address someone as 老板 [laoban]?
Laoban is often used in daily speaking language. 

People working in shop or restaurant often use this to address their customer to show some "respect" or to make you feel good. Like in the car washing shop, they often call you "Boss".

The beginning uses the word was from late 80's, start from the south part of China, maybe GuangDong. 

If you are working in a high-tech or similar place normally don't directly use this word to call your boss. But it's absolute normal use it when chatting with your colleague. 

For University, graduate school students often use laoban to address their director teacher. 

Normally in daily life, you can use laoban to call anyone is in the manager place. Like you can call your partner laoban.