The thongs I use for sport with leggings and work are a mix of polyester and spandex. I want them to last as long as possible.
I experimented a bit with one: I made a baking soda paste, applied it to the stained parts, let it sit for an hour, hand scrubbed it, rinsed in cold water, pressed the thong to get rid of as much water as possible, let it to air dry. Am I being too careful?
Baking soda removed all stains.
Do you only hand wash your undies?
With those materials, hand washing is essentially theater. Machine washing spandex or polyester isn’t going to make enough of a difference over the life of the garment to matter. You’ll wear it out through use before the washing method could do anything.
Some fabrics are more delicate, but those are tough as hell.