No right or wrong answer. Same situation here except we are second floor and the s are carpeted. They are backless.
The 17 pound boy walks most of the time. He does stop to sniff getween the steps though.
I only let him up and down on a leash. Backless really aren't safe for small dogs.
Ellie (6 pounds) is carried bc of her back problems. She used to walk sometimes. The backlessness was a problem though. Be very careful... Never seemed o bother her knees but in her case the LP has never caused obvious symptoms.
BTW, I'm not one to stop the pups from walking (except for medical reasons).