All star resorts no longer share buses.
Unless they've totally abandoned 15 years of practice in the past few months, they do share. But it fluctuates, depending on the time of day and the park involved.
Studios and Animal Kingdom buses will pretty much always share going from park back to the hotel. And going to the park they'll share except when crowds peak, like opening time. (Even then, they'll share more often than not)
Epcot and Magic Kingdom get a bit more complicated, and again it depends on the crowds. Middle of the day they'll share going to and from the park. First thing in the morning it's a toss-up. End of the day rush and they switch to separate buses.
The one good thing about staying at Music is that since it's in the middle, at times when they're running the routes separately, if one resort's line happens to get significantly shorter, you can hop on that resort's bus and walk over to your resort.
And purely from a numbers standpoint, the Music resort as a whole has about 5% less capacity than Sports or Movies, because of the Family Suites. (A room can hold 4 people, a suite can only hold 6 people while taking up the space of 2 rooms.) So theoretically, if every room in all of the All-Stars were at max capacity, there would be 5% fewer people at Music, thereby making their food court and bus route slightly less busy)
-Rob