I don't see any harm in it provided you know what there up to, although when there in friends bedrooms you wouldn't really know anyway. I think its healthy for kids to get some space from their parents, after all its not just them that drive us nuts, we drive them nuts sometimes too!
Its an individual thing, depends on each child and what the friends are like.
The rule in this house is doors stay open but then the kids think they live in a barn anyway

Quite agree with Ellie,

We have an open-door policy in our house (aside from the toilet, who want to see that!!). Seems to go down fine. Kids are all seven.