It really depends on who you are playing and what position you are in. Carries typically rely on their damage to be useful, so they can't really afford to be missing a full item for wards. However, they can and should upgrade their vision trinket to give them Green or Pink wards.
Junglers are a different story. Both the Wriggle's Lantern/Feral Flare and Quill Coat/Spirit of the Ancient Golem give a ward, so many junglers are fine even if they get an Oracle's Lens. Certain junglers (Lee Sin, Nunu, Elise) can get away with building an early sightstone without really harming their impact on the game. This is very good when you consider that the junglers can roam the map freely and ward either jungle or the river to watch the movements of enemies.
Tanky top lane champions (such as Maokai, Malphite, Volibear) can on some occasions get away with building a Sightstone as well. It shouldn't be purchased too early in the game because it hurts your ability to fight your opponent and you don't roam that much anyways, but you can typically buy it after your first or second completed item without issue. You might not get the full cost efficiency of a sightstone rush, but it still allows you to have wards in the late game and gives you some health.
