Say you're travelling on a shoestring budget, say at the end of a long trip where you've spent almost all your money but still have time before your flight home.
Sometimes people you meet travelling want you to go out with them for a meal or drinks, which you could squeeze into your budget if you choose cheap places and the cheapest options on the menu.
But what should you do when this happens in a country where tipping is necessary and expected (due to extremely low wages for waitstaff for instance)?
Is being skint ever an excuse not to tip or to tip below the "normal" percentage? In such circumstances should you always just say "no" to going out and self-cater instead rather than do something which would be rude and/or offensive to staff and/or local people?
(Assume the service was good but not amazing, and if it matters which country, then assume USA.)