As an American, I have grown up with the idea that it is important to tip certain types of workers. People like waiters, barbers, taxi drivers, and bartenders need tips but for different reasons.
I normally cut my own hair, but if I feel the need to get it done by a pro then I will give a generous tip. I tip my barbers to show gratitude for not destroying my haircut with negligence. They hold a lot of power on people will view you.
Barber tip = 0-25% tip depending on result
In America bartenders expect pretty large tips, like 50%. If you go expecting to pay only the drink price then you may not get any service after the first drink. It depends on the bar but I have seen cases where the bartender will refuse to waste their time with a lousy tipper. They get tips based on service, and how often you expect to return to the location. If you want to become a regular you better leave a good impression. (I do not enjoy drinking at bars, due to the large expenses...)
Bartender = 0-50% tip depending on service, and likelihood or return.
Taxi drivers are a different story, If they give bad service or attitude then I won't bother with a tip. If they drive me to my home, then I will tip no matter what. They deal with people all day so sometimes they can be very callous.
Taxi driver = 0-15% tip
Last is the waiter/waitresses. My culture teaches to tip these workers. When we go out to eat, we expect good service. They need to be filling up cups of water, and they need to be very attentive. The tip a good waiter or waitress gets depends on the overall price, and the quality of service they provided.
Waitress = 0-30% if its under like 8 dollars i may be a baller and go 50% tip if they were really good ;p
Before I go and buy any services like these, I have to ask myself " do I have enough money for the service, and to pay respect to the worker?". This is key and not everyone thinks like this. A young person who becomes a professional athlete may not have grown up with these ideas. They may be more focused on the idea of buying material items to highlight their wealth. If they had almost no money and then started to receive very large amounts, how well could they be expected to handle it? They may buy a mansion, a nice car, lots of clothes/tattoos/jewelry , and also treating their friends/entourage to nice meals. A person living like that would probably not tip because that is just one more expense they do not need. A person who does not tip on big meals should not return , because the workers might remember them and spit on their food.
I do not think people need to focus on a persons salary and how well they tip. The person with the money should know how much they have to spend. In America, we tip certain jobs, that is just an underlying expense that is expected. If you are famous then many people will have their eyes on you and all your actions will be scrutinized.