The problem with 'To each their own' is that it often only applies one way. In the case of assistance service, it probably seems fine to an American to tip when in Europe even though it's against local practice, but then is it also OK for a European not to tip in the US because that's their normal behaviour?

If I've learned one thing through travel it's to take the time to understand local culture and customs and try to observe them. It's not about trying to pass myself off as a local, but merely respecting the places I visit. Treating foreign people and places as you would back home isn't a good default.