With the current fashion for tight lycra belly dance skirts, it's becoming common for belly dancers to wear a G string– but it may not be such a good idea, because belly dance costumes are probably more prone to "wardrobe malfunctions" than any other kind of dancewear. Ballroom dancers, ballerinas and skaters may look as though they wear brief costumes, but get up close and you'll find they're not showing as much skin as you think!
A Latin dance dress may appear to be a two-piece, but in reality the sequins and beading are applied to a flesh-colored base, and often the whole torso is covered. Even if there are cutouts, the top and bottom are always connected so there's no danger of the skirt falling down. The example on the left has mesh connecting the two halves. Connections like this are much less common in belly dancing costumes.
There are belly dance dresses (sometimes called baladi or beledi dresses, or galabeyahs) which are all in one piece, often with very large cutouts – but most belly dancers still wear a bedleh (bra and belt) or bra and skirt. That's true even in Egypt, where bellydancers must cover their midriffs – most dancers simply add a body stocking to the standard bedlah.
So, a typical belly dancer relies entirely on a zip to keep her skirt up. And given the skirt sits very low on the hips, there's always the risk it may go beyond the bounds of modesty! As if that's not enough, most belly dance skirts also have at least one split, sometimes very high. It may not reveal too much when standing still, but dancers don't always check to see what's exposed when they're in motion. As the split separates with leg movement or fast turns, you may be surprised what the audience can see...
A circle skirt is even worse, because they can fly up when turning. Chiffon skirts can be extremely see-through in the wrong light - but even a heavy skirt can show more than you think. Flamenco skirts are voluminous and heavy, yet I have photos of me in performance with my legs exposed to mid-thigh!
All this means you can't assume your modesty will be adequately protected by your skirt. So the safest option is not a G-string (which is going to display far too much of your nether regions if the worst happens), but a pair of dance pants – ideally in the same shade as your costume. If you get them granny style, so they fit under your buttocks instead of cutting across your butt cheeks, you needn't suffer from VPL (Visible Panty Line). You may even want to go for dance shorts or bike shorts, which have short legs extending down the thigh to give a smoother line.
Personally, my current favourite is shapewear. I don't buy the expensive kind: instead, I buy the cheapest I can find, and (this is important) I buy a size bigger than I would normally wear. Then I remove the "grippers" around the thighs. The result is a pair of skin-coloured, figure-hugging shorts that give a smooth line but (because they're a bigger size) they don't make me feel constricted.
Granny pants may be boring, but on the whole, it's a case of "better safe than sorry"!