To be a competent support above Gold, Sightstone & Control Wards are an absolute requirement. The time at which you BUY a sightstone can vary by game, generally based on Jungler/Mid enemy champions - but you should almost always get one.
If you do not, your putting your entire team at an extreme vision disadvantage. This will lead to more lost games. Additionally, enemies who are good WILL be very aware of what your buying, and if they know you have no vision, they will roam bot much, much more often because your essentially playing blind. Easy pickings.
Vision is the most important thing in league of legends. Always buy a sightstone as a support if you want to climb. You can get away with it in Silver and maybe through Gold if your a strong player, but after that, heh.. goodluck.