As I understand it, there are certain sections of the 50's, 60's and 70's that are preferred. It used to be the 60's only, but now they just have certain locations of those three areas. I would reason it would be rooms in those buildings closest to the main pool/food court. The buildings are T shaped and pretty large so if you look at a map, you can see that certain wings are much closer to the Hippy Dippy pool than others.
I hope that helps, I can't ever get the search function to give me what I need!
Okay, here is some information I got from the Pop Century FAQ thread:
Preferred Rooms
"Preferred" refer to the location of the room on the property and nothing more; they do not refer to views or amenities
these are rooms in buildings closest to Classic Hall (500 ft or less); this does not mean your room is necessarily THAT much closer the bus stop, however; see "Property Info" section in the following Post
typically $10-$15/night more than a standard room
these room numbers are considered preferred rooms (note x defines the floor)
Building 1: 1x01 1x36
Building 3: 3x61 3x72
Building 4: 4x01 4x36 & 4x61 4x72
Building 5: 5x01 5x12 & 5x37 5x72
Building 6: 6x01 6x12
Building 10: 0x37 0x72