Well, I think I am the one who mentioned the shirtless thing in the previous post. So you obviously have some big clues as to what my opinions are. :)
I DO have both a boy and a girl in my home. My son is the older of the two. My husband (personal preference) has never walked around naked around either of our kiddos. Neither have I.
We have walked around in our underwear (and bra for M.). However, I no longer walk around in bra/underwear in front of my son. So, yes, I think the opposite sex thing comes into play at some point. Every family is different about what level they are comfortable with, but growing up in my household (I had two older brothers) I never went out in public areas of the house without clothes, be it a bathrobe, PJs, or whatever. I never did the bra/undies thing. Neither did my parents.
My husband does not allow our son (almost 14) to walk around the house in only his underwear. And I think that is appropriate. Again, OUR opinion. Our daughter is almost 11. By age 9, she would have been mortified to have someone open a door on her without a top of some sort. Or bottoms.
Our kids do not thing there is anything "bad" or any reason to be "ashamed" of their bodies. But, they do recognize that they are private. They understand there are certain parts of their bodies that are not proper to expose in public. And public doesn't J. mean out at the mall or whatever.
My daughter is at the age where she wears the little "bralette" things, but that is a development from the past year or so. But well before that, she wanted her privacy.
In our house, the need for a certain amount of modesty and privacy arrived at about the same time that the bathroom door needed to be closed to go pee. There was a point when I didn't need to see and didn't WANT to see that. Also for them, there was a point when they didn't need or want M. TO see that. THAT is around the time when we started requiring son to have on more than J. underwear, and daughter to have on a top.
At 5, no, there isn't much difference (well none physically/visually) between a girl and boy's chest. But I feel it is important to instill a certain level of modesty in my daughter. I don't want her flashing inappropriate levels of skin when she is 15.
And yes, if a bathing suit covers it (bikini for a girl) then I think at age 9, they should be covering it around the house.
In some families it may be different (heck, some people are nudists), but, in my family, it is inappropriate for a 9 year old girl to not put on a tshirt or robe to walk through the house when there is a male present. Whether brother, dad, uncle, grandpa or next door neighbor (unless the brother is a 4 yr old or something).