6th item as support ? Or always keep 2 control wards

why waste gold on wards? no one pays attention to them

Generally best to keep a slot open for wards, but if it gets really late game to the point where 1 team fight will be the deciding factor, you'd likely be good to buy that 6th item (as well as sell boots for a combat item) to give you the best chance of winning that team fight. It's your own judgement call on whether vision or combat power would be better in that specific game.

it goes down to what you firmly believe will win you the game. Over all though in fives play, leaving it open is superior.

I tend to leave the 6th slot for 2 control wards until I have enough gold to buy a 6th item outright. In my opinion, no component item is more valuable than control wards. I tend to only value a 6th full item when the game has gotten past the 2-barons part, since that's when the game tends to be determined by winning a teamfight rather than securing baron/elder.

Is having vision in a spot where people may or may not check that bush. I find it that if the enemy team isn't checking for wards, then they most likely aren't that good. Most good teams will have 1-2 sweepers on the team anyway so spending that gold on an item that could be the difference. If you are at 6 items. You prolly need that last item. Example I think would be wards around baron or drag. but if it's not then you are looking for the pick, or should be. My advice if you need that control ward get your best duelist to split and bring someone to that lane.