I wear flashes all the time. I think it finishes the look. Of course I should also say that I wear traditional kilts, all but one are tartan (I have one solid black). I always wear a belt, hose and always (except inside) wear a sporran and sgian dubh.

I wear flashes, pretty evenly split between solid color and tartan.

If you are going for the "traditional" look, I would recommend wearing flashes. They keep the hose up, and ensure that the sgian dubh doesn't fall down also. With a very little effort, you can whip out some very nice looking flashes, and with a little extra effort, you can make some really nice ones.