It seems possible that the rule (or at least a tendency along those lines) existed at some point, but this would be the kind of thing that tends to change over time.
While doing online research, came upon this thread and website. Glad my commentary on 4 was of some use. Can confirm that Aeneid 1 is about two-thirds done in first draft; plans to do 9 first were changed to address the more pressing textual and other problems of 1, as well as to finish the “Dido” books.
I haven’t checked it out, but I thought I’d post it since it seems like an interesting approach and a good resource for beginning/intermediate readers.
The Lover's Curse: A Tiered Reader of Aeneid 4 (Latin Edition) [Hurt, Carla, McCormack, Jessica] on Amazon.com. *FREE* shipping on qualifying offers. The Lover's Curse: A Tiered Reader of Aeneid 4 (Latin Edition)