It's more a question of what you want them to do or not do.
In reality I have spent very little time in formal training and still successfully pack with them and am happy with their performance.
We don't hike nearly as much as I would like to. We don't really condition. I don't teach them to load. They just go with me where I go, so I put a pack on them.
We're not going to win any contests at the rendy, except maybe the canoeing one. But if you want your goat to fetch beer, he can probably learn it.