i wear them under shirts...i sweat alot so its a must. usually just lounging around the house otherwise
when i was more jacked i would wear some cool sleeveless shirts...they were great shirts, just happened to have no sleeves.But never really a straight white tank top.
i think skinny girls can look really hot in wifebeaters (with some type of design) if the rest of their look coincides with it.
how the rest of you looks plays a big part of if you can pull one off. If you're a white pasty dude, you'll look like you walked out of a trailer park. If you're dark skinned, dark haired, don't have a lame haircut, and are in decent shape it can look a little different and better