While I am aware of Corgis, I've never owned one so I'm not sure about this. I think going up should be just fine, but if you wish to be absolutely certain, consult your Vet. I am quite sure that would be a question happily answered over the telephone.
Just from a Behaviour stand point, if you don't allow the dog to learn about the stairs before a year of age, they will become a very frightful obstacle. I'm sure the Breeder is concerned about back issues on their growing bodies, but we all know that going down is much harder on the body than going up, for dogs especially.
You sound like a very caring owner, so I would suggest you follow your own instincts. Perhaps read some articles, or even purchase a book (I'm old fashioned that way) on Cardigan Welsh Corgis and see what the experts say. A lot of Breeders have their own little peccadilloes.
Our Behaviour Section might be of interest to you, as well as our Health Section as many different topics are addressed in both. You might even wish to start a thread in either of those places as it will more likely be seen there.
Good luck to you!