I really don't think it matters. I think they will be delighted with any tips. With the Jade in Europe year round they get both. The ships currency is USD and many of the ports if they get off the ship take Euros.
When I was on in July many of the crew were not happy to be in Europe. They were looking forward to spending half the year in NYC were tips are common. That as we know is not happening. Different cultures different tipping practices. I will leave at that and say one more time they will be delighted with any tip.