I'm trying to organize my trac information better. The trac data model has the following elements:
I was wrestling with what milestone and version meant with regard to each other, so I reviewed the Edgewall (the makers of trac) trac.
If I understand correctly, versions correspond to tagged snapshots of a subversion repository, and most likely packaged up for distribution.
Milestones are versions planned for the future which have not yet been completed.
Along those lines, I think it makes sense to only put tasks in milestones, though I'm glad that they allow the freedom to enter tickets any way you want.