Take a look at this page from the FCC website. Under "Content of Signals Subject to Mandatory Coverage", it states that the station gets to choose which subchannel gets carried by cable or satellite. Most stations would carry their most valuable signal on the -1 subchannel, and therefore most likely to be carried on cable, but it's not mandatory.
http://transition.fcc.gov/mb/engineering/digital.html
http://transition.fcc.gov/mb/engineering/digital.html