I just came back from a recent trip in Guam and, as a French person that is not used to this habit, I didn't tip in taxi, restaurant,...
Is it customary to tip in Guam?
Sure, it is a (non-incorporated) territory of the US, but I felt that the culture was quite different to the one I experienced when traveling in the mainland:
- Staffs were always very friendly when I was leaving a restaurant (contrary to cold greetings when I forgot to tip in the mainland).
- No one made the remark that tipping is customary here (happened to me once in Vancouver - I know it is not USA, but Canadian habits are similar on that point).
- When paying in cash in the mainland, I was often asked how much I wanted to pay, meaning how much tip I wanted to give, before giving me my change. This never happened in Guam.
- I found staffs very 'present' in countries where tipping is the norm. But, except one "is everything fine?" after my order, I could eat quietly in Guam.
- Many places, especially in the Tumon area, cater to the numerous Korean, Chinese and Japanese tourists (e.g. they only had Korean menus), countries where tiping is a rarity.
- Prices were all tax included (from the answers to this question, I guess this is because Guam administration is simpler compared to the mainland). This is not related to tips, but it shows that the local culture is quite different.
In short, from all these 'hints', I never felt it was a mistake when not caring about tipping on the island. Do I have the right impression?