Lots of variables here. One that I haven't seen mentioned is, how early do you want to be on the ship? Boarding early can be great. We took a disney cruise and the kids were excited about the big water slide, so we did OLCI as soon as it opened to get an early boarding group, and they rode like 10 times before it got crowded. Sometimes those first couple of hours of having an uncrowded ship can be worth it. Had to go the night before.

The other ones that matter, I think, are where you're coming from, frequency of other flights, and your cruise's first stop. We are doing a cruise from CPH. If we were flying from the USA, I would go at least one day early, and probably more. Jet lag is rough on a port-intensive cruise. But we are going to Paris first. To me, having that last night in Paris and only packing once is worth the potential stress of a day-of flight. It's a short flight, and there are other scheduled flights. If we miss the ship, the first port is in continental Europe the next day, so we could get there if we had to. I generally need to buy trip insurance for cancel-for-work anyway, and my policy had a cheap add on for missed-cruise insurance (transport to next port plus meals) so long as we are booked on a non-charter commercial flight scheduled to arrive four hours in advance of cruise departure.