I would say that, in the laning phase, it's the jungler's job to ward their own buffs, and each lane should ward for themselves (mid in the river bushes and top outside of the river bush.)
When it comes to teamfight time, everyone should still be buying wards, but the ward placement simply changes. What would be best is if everyone was simply aware of the baron ward and, when it came time for it, whoever was closest warded. But as this isn't premade 5 and buying wards seems to be beneath most normal players, it will usually end up falling to the support to do. I say, if you see that it isn't happening, take it upon yourself to do it.