A good rule of thumb is that a sgian dubh can be too dressy for casual wear, but (a well made one) can never be too casual for formal wear.

What I mean is this: A sign dubh designed specifically to be formal or fancy looking (lost of silver work, stone cap, etc) might look fabulous with formal attire but a bit over-the-top with daily dress.

On the other hand, a relatively plain looking staghorn or blackwood sgian dubh would look great for casual dress but will also work marvelously for formal wear.

Look at the Oxhorn sgian dubh (4th one down) on this page:
http://giftshop.scottishtartans.org/blackknife.html

For the longest time, this was my one and only sgian dubh and I wore it for all occasions.

Aye,
matt