Obviously it depnds on where you are cruising. When I sailed the Mediterranean out of Barcelona, I planned to go 2 days early, but ended up with a half day in Amsterdam airport, missed connection, and I planned 2 more days at the end, but got too tired and flew out the day after. With cruises out of the US, I almost always fly in a day early, relax before getting on the ship.
When I first cruised, I always had the agent book with the cruiseline, but now with the internet I save the money like regandy for spending. I book the flights myself, knowing that I am responsible and take the chance. But nowadays I more likely to buy travel insurance too