OP reporting back. Thank you all, fellow FTers, for the very helpful comments and advice.

At first, I thought my question was simple, but the experience of others shows that there are several levels of response, depending mainly on local culture and attitudes. In our case, our next trips are within Europe and to South Africa. I'm still hoping that a return to the US is not too far away - I'm stuck at 37 states, and need to visit the remaining 13 and revisit others. And hopefully back to NZ and Oz.

So for us, your comments have firmed up my thoughts:
- within Europe (likely to be Northern Europe and the Nordic area): happy not to tip
- Australia/NZ: ditto
- South Africa: tip
- US: tip

Thanks again.