Remy never really had any problems with the stairs, one day he just did them. Well actually no, I'm getting a head of myself, let me start over.
When Remy was about 10-11 weeks (I think) he could go down the tiny flight of stairs, which consisted of 3 steps, from the kitchen to the living room. A couple of days later, he got the hang of going up the 3 steps. A day or two later, I was upstairs relaxing and he wanted to be with me, so he braved the normal flight of stairs. The only time he's ever hesitated was when there was stuff sitting on one of the steps, other than that he's been find. Even when the Copper snapped at him and he fell down the small flight, I didn't make a big deal of it (even thought I was freakingout on the inside) and made him go up so he wouldn't be scared of the steps. If he did contract a fear, I would probably use some primo treats to help with a good association, and help him forget about the fall.