If you have more kids than adults (like 2 parents and 5 kids, all under the age of 18?) then WDW will try their hardest to get you connecting rooms. I'm not sure if they use the word "guarantee" on the reservation, though.
If you have more adults than kids, they will note your request, and try to fulfill it, but it's not a priority for them. That said, we have always gotten connecting rooms when we have requested them. Once we had a block of four rooms, when 18 of us went down together. Once family needed a more accessible location, but 14 of us were right next door to each other, with two sets of connecting rooms!