You will get a lot of opinions on this. Some people like to get there as early as possible so they can have the most possible time on board. This means more lines. I prefer to arrive around 12:30, breeze through a mostly empty terminal before the WDW buses start arriving, and head straight on board. No lines, no stress. So it's really a question of, do I want every possible minute on board even if it means a fair amount of waiting around in lines, or do I prefer a more relaxing start to my vacation even if it means I lose an hour or two onboard? That's a personal decision.
Remember when you get on board is related to the port arrival time you pick at check in. If you get a late PAT, there is really no point in getting there super early as you won't get on board until after those with earlier PATs do.