I'm not sure what's customary, but this is what we did. We had the pleasure of eating in Palo's on all 4 nights of our cruise. I was unsure what to do about tipping our regular servers, who we never met. I called and was told that we should give the tips to Palo's instead. We had been tipping our server there nightly anyway. Then I thought about the breakfasts and lunches that we ate in the regular restaurants. We didn't tip the servers at those meals and maybe they were the same waitstaff. I ended up tipping the serving staff the recommended amount and I was comfortable with that.
Not sure how much that helps, just my experience with the situation.
Tammi