Flashes are perfectly acceptable for casual, semi-formal, and formal wear. In other words, all occasions.

As others here mentioned, unless you want to be pulling your sagging kilt hose up all day, some kind of garter is necessary to keep them in place. Some people here have discussed making their own elastic garters sans flashes (or you could just take the flashes off of a set of garters you already have), but I personally don't see a need to go through the trouble. Flashes are neither casual or formal wear -- they are just part of any kilt outfit. So feel free to wear them any time.

M