I would never put on a pair of briefs if there is a chance that a guy has had them on before me.
Most Helpful Girl
Swimwear, yes, but over your own underwear. At least that's how you're SUPPOSED to do it, but I'm sure there are some oddballs who don't care to follow that little bit of etiquette.
I've never tried on underwear and I wasn't even really aware that people do this, haha. But apparently they do. I just know what size I wear, though I can usually wear a size smaller as well, depending on the individual pair. And you can usually tell if underwear will fit just by eyeballing, it if it's your size or a size up or down, depending on the style and material. If it doesn't stretch much, you stick to your size. If it does, then you have a bit more leeway.
Bras, however, should always be tried on. I'd never buy a bra without trying it on first because I'm very picky about how they fit. All cups are NOT created equal. =P Give me ten different bras, all in my size and in styles/designs that I like, and I still may only like one or two of them after trying them all on.0