From my observations, I think it depends on the person & their financial circumstances more than their religion.
It's not customary to tip where I live but depending on what group of people I am out with depends on how tight they are. Most of the jw's I socialise with normally give a little extra, especially when the service was good. One of my closest jw friend & her husband are very generous & giving whether it's paying for yr dinner or a drink, and giving gifts, not cause they have to I.e. Birthday, Christmas, but coz they want to. its their ways of thanking those that mean a lot to them. But they are only that way towards those they consider their real friends. Btw - they are generous to both their jw & non- jw friends. On the other hand, I know some, both jw & non-jw that are so tight with their money it becomes embarressing. I have seen that behaviour with both jw & non-jw's. I have seen some (both jw & non-jw) who are normally generous with giving who have had to suddenly watch every penny coz of losing their job so have had to become 'tight' with their money to pay for necessary expenses like their mortgage and utilities and other everyday expenses or they are saving up for their dream holiday or a house. I have also seen some (both jw & non-jw) that are wealthy but they are embarrassingly tight with money...