I definitely wouldn't surprise them with it and I'd talk about it before it doing it. I also think there is a significant generational issue, older people tend to look on it more negatively I think, I had no problem talking about it with my wife's sister, but I'm not really interested at the moment in telling her parents or even my parents, although if had to pick one I'd choose my wife's dad over mine because he seems a more accepting type. Telling my grandfather is about the last thing I do. I don't really see my older family members enough to where it would significantly cut how much I dress, so I don't think it's really worth making waves over.
