Jekyll,
probably what everyone is saying is true, but that's because most people here tend to have annual passes or some kind of length-of-stay passes, so they want to take advantage of every last second of time in the World they can. However, we don't know what kind of passes you have for the parks. For instance, some people who plan on staying in WDW for a week only get passes for 5 days, assuming they might want to rest for a day or go to Universal or Sea World, etc. You may have to choose the days you spend in the parks, rather then go whenever you want. And if that's the case, you might not want to go to the parks your first day, especially if you're tired from the flight and worried you might not enjoy yourself.
Despite that, however, there is no downside to checking in early. Depending on what time of year you're going, you can get on the airplane with shorts or swim trunks on underneath baggy pants or sweat pants. When you check in, go poolside, or hit a water park, etc. You can grab something to eat to get the stank of airplane food off your breath. You can go shopping at Downtown Disney, or with a rental car go to local outlet stores where the prices of Disney swag is much less. And of course, if your passes include admission to the parks that day, go nuts anywhere else you'd like to be. When you get back to your hotel, your room will be ready, your stuff will already be inside, and you can do whatever else you want to do.