This is what I was taught. The sgian dubh was always a hidden blade, when entering a friends house you would remove the knife from its hiding place to show you were friendly. When you were seated the knife was placed in the top of the stocking by your strong hand. This was done so while seated you could, if required, draw the knife quickly. As for the dirk, I was also taught to keep it on the side of your strong hand. I don't see many people wearing a dirk these days, except for Burns' Nights or other formal Scottish galas. Just my two cents worth.