As this is a Princess board, I will confine myself to what we do on Princess. We have Princess Plus, so all tips are taken care of. We normally then tip our steward, and, if we have the same servers at certain bars or restaurants, we will tip them at the end of the cruise.
But, I'm sorry, if I walk to a bar, and the bar tender just pours me a glass of wine and hands it to me, that, in my view, does not justify a tip.