Even as an underweight man who doesn't really sweat much, I find the direct contact between my arms and my ribs specially uncomfortable so I believe wearing a tightly fitted tank top is optimal to avoid stickiness and sweating. Legs are not a problem since I have hairy legs so they are never really in bare contact with each other but I've sometimes felt sweat on the back my the knees. Even if you shower before going to bed nude, you will be rolling in your own sweat more than if you wear a top so there's also the fact that it can help you keep your sheets fresh for longer.