I have always been open about dd's sn as I feel that it's not her fault and it does make life easier if I explain (obviously I don't broadcast the fact either!).
I am not sure how common this is as I know quite a lot of people on the "sn circuit" where I live who will only say something if they have to.
By this I mean if you are in a shoeshop etc As dd plays up in shoeshops I do tend to say she has sn and it makes life easier all round.