Hi Ben.
In general, the collar tie's job is to keep the rafters from sagging - not to keep them from spreading. As Joe points out, the roof pitch, and the snow load have a lot to do with how much you can get away with from an engineering standpoint. Another crucial factor is the width of the building.
We build this design fairly often, and our favorite way to mitigate the rafter thrust (and take the stress off the post-to-tie beam connection) is with queen posts to the collar tie. Now the collar is lifting the rafters, not just keeping them from sagging. In this example, the roof pitch is 12/12, the snow load is considerable (North Shore of Lake Superior), and the building width is 28':
