Restaurants always, so long as one (or more) servers is obviously assigned to serve your food. It is figured by everyone that tips averaging 15% is part of their wages. But adjust that up or down from 15% to thank excellent service, or emphasize poor service.
Likewise in a bar if the service is as good as you might hope, otherwise you might just leave the odd change.
But not in a McDonalds-type setting, or a fancier 'all you can eat' cafeteria where you carry your own food to the table. In a fancier cafeteria you might leave a small, and optional, tip if someone is obviously assigned to your table and has been unusually attentive filling water, clearing dishes between courses, etc. In many cases there is a crew of real stupid kids in that role, no tip necessary.
I seldom have anyone handle luggage, so seldom tip for that.
Everyone knows that foreign tourists are unpredictable for tips so don't feel bad if you later realize you slighted someone. In fact Canadians are notorious for a never-tip attitude, they almost make a joke of it.
15% is figured on the cost before taxes. Around here there is usually about 7.5% taxes added to the bill so you can double the tax then /10 to estimate the tip; adjust slightly for differing local tax rates. Close enough.
Tipping is the custom only in the dining and lodging industries where serving you, creating a friendly atmosphere, is part of the entertainment.
Never, in retail shops.
