I like all piercings (tongue, nose, ears, lip, etc) as long as there isnt too many of them. Some girls pierce everything, and their face ends up looking cluttered and ridiculous. That being said, I like nipple piercings, as long as they are fairly simple piercings, like a bar or ring and maybe a little chain attaching them.
Nipple piercings are often associated with bondage, which im a big fan of as long as its happy/consentual bondage. And in hentai there seems to be a lot of images of pregnant girls with their nipples pierced. I'm also a big fan of pregnancy hentai, once again, as long as its happy/consentual.