I don't believe in knives that are decorative and not functional. Where I live now, I can't exactly go down to the local knife store and get a sgian dubh, and I am weary of ordering a knife on the internet because you can't really tell from the descriptions and pictures. Therefore, at the moment I don't have a sgian dubh to wear formally or casually.
My parents are currently having a sgian dubh custom made for me back home. Once I have that, I will wear it in my sock casually as well as formally because I always carried a knife when wearing pants and it wouldn't make sense to stop because I am wearing a kilt.