We usually prefer to board earlier, just to maximize time on board; getting on earlier gives time to eat a reasonably leisurely lunch, walk around on deck a little, and then check out our cabin soon after it becomes available. When kids were younger, there were some check-in things that we wanted to do earlier, as well.
An early PAT also gives flexibility if you're traveling beforehand - if you encounter a problem and arrive late, it's still OK. If you have a late PAT then arriving early is not good, but arriving late runs a risk of missing the ship.
But, the main thing for us has really been that there's not much reason to not board sooner. We've twice stayed in Port Canaveral - there is not that much to do in the morning before boarding and you have to check out of the hotel anyway, so why not board sooner? We stayed in Orlando once, and it's only a little over an hour away - not much difference in leaving at 10 vs. leaving a couple hours later, for us. And, other times we've driven 2.5 hours from home to Galveston; leaving around 9 was not any worse than leaving at 11. But, if you had something you were going to do with those extra couple of hours, then sure, the later time might be just as good.
We have had a cruise where we arrived later, and the advantage of that was that there was no crowd/line, and we basically just breezed through all the check-in/boarding. By the way, this worked at other places, too. I remember visiting the Vatican museum at a time when people stood in gigantically long lines and lined up more than an hour before opening in the morning - we arrived after lunch and had no line at all.
