Taxonomy terms attach to nodes, not to revisions. Naturally, you don't want to lose all your tags and categorization when a node is edited, but it's worth noting that adding or deleting terms does not create a new revision, that nodes do not keep a history of terms applied to them.

There are differing opinions, and I'm a little scared to look at the guts again after reading this issue, Revisions and taxonomy don't play well.

(A really old issue on the subject: suggestion for saving taxonomy term selections on rollback...