In the US, I've always seen that the "unofficial" tip amount of the bill is 10% to 15%. However, I'm seening now that most people are tipping more like 15%-20% for getting very good service.
However, I find it funny that a lot of restuarants are "printing" suggested tip amount and their corresponding percentage on the bill. Like amounts for 15%, 18%, & 20%.
I also have seen some restuarants automatically include a 15% tip regardless of the size of the dining party. I think for thw wait staff, this really lmits their tips because most people don't like being "told" what to tip and won't tip more. It also hinders better service because the wait staff won't work harder since they know they typically won't get more tip for better service.
Some countries the tip is included in the cost of the meal. IF you've gotten exceptional service you leave a small amount extra. I think this is true for France. I have no clue what they print on the check/bill since I don't speak/read French.
In Aruba, they add 15% "service charge" to every bill. However when they charge your credit card you still have the spot to "add your tip" on your charge card. Many people there don't realize the tip has already be added and the waiter gets an "extra" 15% tip (average). Considering the average entree for dinner in Aruba is $25 to $35, they rake it in when "us tourists" don't pay attention.
Alcohol? I see no reason not to tip for Alochol. They provide service just like food waiters do. If you get bad bar service tip accordingly.
Gas station attendent? When's the last time you bought gas in a "gas/service station"? Most of them are "convience stores" now with pumps out front. It's very hard to find someone to pump your gas for you. I doubt most people tip the person behind the counter when they have to go inside to pay. Or they just "pay at pump".
I will say that when I get good service at a place where tipping is common and I tip them good, that the servers/etc remember that and remember you when you come back in. Friends who work in the "service industry" supported by tips say that they remember good tippers also.
Other things I tip for: When I get good service from hard working Parking attendants/valets, the paper boy, car wash employees, boat crews, sky-caps (airports), hotel valets/bellboys, delivery drivers.