Okay, well, we pay a very nice lady to make dresses that fit properly so that the entire crotch area is completely covered, regardless of leg position, and she uses elastic that doesn't roll or pull aside. I have seen other dresses where the crotch is so narrow that it is potentially indecent should the skater not have additional coverings. At one point, the international competition videographers were directed to avoid shooting up skater's skirts due to potential issues.
Now, undies. Commando for the girls, because they find it more comfortable not to have another layer of fabric shifting about, and neither care for thongs or high cuts, and both dislike panty lines. And, frankly, by the time you layer undies, tights, and snug fitting skate bottoms, there is - chafing - potential.
We have a very famous pic of our elder DD skating in a comp - on a Sunday - and you could clearly read "THURSDAY" on her day of the week panties. For comps, they wear lighter weight tights than for daily wear, and there is no way panties could NOT show through.
I do think though that whatever the choice, the skater should do runthroughs with the undergarments of choice to avoid surprises on competition/test day. If a skater does wear undies, then, a pre-skate "butt check" should be done (bend over, touch toes, make sure all is neat and tidy and in place).