When you live alone, there are things you're perfectly comfortable doing, things you don't think twice about. And then, one day, you live with someone and they happen to observe you brushing your teeth in the shower or eating last night's pizza for breakfast (pizza that sat out all night, of course), and you wonder, "Is this gross?"
If you're lucky, your roommate or partner won't think it's icky, and might even do the same thing themselves. Then you can discuss how weirdly fun it is to brush your teeth in the shower - bonding! (Though it may mean you have to share the leftover pizza.) But it's too much to expect everyone to agree on every quirk, so gently establishing what the other members of your household find gross can help avoid resentment, annoyance, and an eventual "Please stop doing that because ewwwwww!" blow-up.
The most entertaining way to broach the topic would be a viewing of the Sex & The City episode in which Charlotte deals with Harry's tea bag habits ("A Woman's Right To Shoes", Season 6 Episode 9), but the best way is probably just to be aware of your own actions and ask about things as they come up. If you're drinking the previous day's coffee, cutting your nails, rinsing a dish you need rather than properly washing it, or enjoying a nice floor picnic, you can casually ask, "Is this gross? Or not gross?" If they find it gross, the habit is probably easy enough to avoid (or avoid doing in their presence) in the future. And if they don't find it gross? Proceed with enthusiasm!
(Image: Flickr user John Tornow licensed for user under Creative Commons)