I feel it's good excersise for them and training. But with anything that goes up, it must come down. So the risk is to take the stairs when you're not around and thumbling down and hurting itself. I would wait when he's older, not sure when but his bones are still developping and taking the stairs too often might cause a strain. But practicing a few times, supervised is great.
Carpet stairs are better for traction or even if they slip and hit their nose. But Wally's not a running in stairs and even with the cats around he doesn't run after them towards the stairs.
Up should be fine in moderation. I believe the worry is all the weight on their shoulders as they come down the stairs. We limited any stair activity until they were 6 months old or so.
I have not had to work with a corgi puppy learning stairs but I can tell you that uncarpeted stairs are a real risk coming down. We really limited our Irish wolfhound pup on the stairs...it was like trying to teach a colt to go up and down stairs. Once he was full grown and could do stairs he loved to lay on the landing...which was so much fun trying to get around him with a laundry basket....not. One day he was coming down, I don't know if he was going to fast or lost control of those long legs but he slid down the long section on his belly and went head first into the closet door across from the bottom of the stairs. That was the first blow to his head, the second one when he slid on the kitchen floor and put his head thru the end of the island started him off on having seizures.
Lucy had some supervised practice with going up carpeted stairs, and as she got older going up other kinds of stairs (hard to avoid when I live in a building with many stairs). But it took her a while to "learn" how to go down the stairs. She eventually got it when she followed a friend's Corgi, but she was about 10 months by this point. At 3 years, she has no trouble with either but it is not excessive. And carpeting on stairs does help (it's the only carpeting in my apartment).
We have one set of stairs in the house and they're carpeter and he doesn't go up & down at all, I carry him down when it's time to go outside and he went up the stairs when he was very excited one day without even realizing it and when he was all the way upstairs he looked down like "wait a minute, I just went up on my own!" since then he always runs up the stairs when we come from outside (I'm always right behind him if he slips).
I am more concerned about his joints but I guess it's always better to be safe than sorry so I'll just carry him up until he gets too heavy :)
Thank you guys