As has been previously mentioned, most rides have mainstreamed lines now, especially at Animal Kingdom, Hollywood Studios, and Epcot (although I think Spaceship Earth still has an alternate entrance).
The Magic Kingdom is hit or miss in terms of lines and boarding. Some lines are mainstreamed, but some have alternate entrances. I highly recommend picking up the special accessibility guidemap when you enter the park. We used to not bother with these but they make a big difference in knowing where to board. Our last trip, we didn't see them displayed with the rest of the maps, but you can get them inside guest services.
Anyway, there are a few MK rides where you might get on faster than if you had waited in the regular line. On our last trip, that happened to us on Jungle Cruise. Then at Small World, we saw people just walk on the ride and we waited 20+ minutes for the wheelchair boat. The same thing at Winnie the Pooh, where we got fastpasses. Just as we got the the front of the line, the wheelchair "Hunny Pot" went by and we had to wait off to the side until it came around again.
Hope you have a fun trip!
Mary