As the previous posters have said, you may need at least the elastic garters to hold the hose up, although that will depend on the hose. I have found that some hose will stay up fine on their own, but some don't, especially if they are older or stretched out. I happen to like wearing flashes with a tartan kilt; it just seems like part of the outfit.

But to simply answer your question, the flashes are not necessary for wear with the kilt, although you may want to wear the garter anyway to hold up the hose.