If you are any good at Sona, Nami, Taric, and Lulu, you should never rush Sightstone on them over Philosopher Stone, because the only way you really should be dying as these supports is if you get ganked, which honestly will get anyone killed if it's executed correctly.
Just general rule of thumb. Other supports it's more debatable.
When can afford sightstone, always get it. Otherwise it depends deeply on match-up and how you're doing in lane.