It's generally not necessary. If you get checked in at the 24-hour mark you almost certainly still will be able to get seats together. Frequently you will get 'A' group boarding passes when you do check in 24 hours before your flight. Even if you get boarding passes in the B group, you generally can find a few two- and three-seat blocks available close to one another. They may be farther back in the plane, however, so if sitting up closer to the front is important, you may want to consider EBCI.
One caveat would be if you are flying out of one of SWA's busier connections--BWI or MDW, in particular--is that you may find yourselves with higher boarding numbers AND in a situation where there are a number of passengers who started on the same plane in another city and will be on board and in seats before anyone at your airport gets on. In those specific cases, seating can be tricker and you also may want to consider EBCI.
IMO, the biggest benefit to EBCI is avoiding the need to figure out some way to check yourself in on the way home. Not that it's very difficult, but it's just something I'd prefer not to worry about while I am at WDW.