I stumbled over my words when I was describing the problem with Caitlyn
getting caught out while Leona was warding, and I want to clarify: If Cait wants to be CSing
and the lane is pushed far up to your turret, they're going to be able to push
her away from the creeps very easily. So she can get away with not
getting killed (by hiding under tower), but if she's not
doing that then they can kill her while you're gone. If the lane is
pushed out somewhat, then (a) the creeps will be more spread out, so she'll be able to take some CS more safely with her support gone and (b) you can return to her more quickly without
being unsafe (by walking directly down river). In either case, the ward at dragon does not have to be quite so far out because Jax's gapcloser is fairly shortrange.
This does NOT mean that it's okay to engage on them without a river ward at all; you still need to see him coming from river, and if the enemy jungler were a Rammus or Hecarim or someone who can close long distances quickly, then you need a deep river ward to be safe. It just means that you should make sure your carry is actually safe when you go to ward (ping him back as you leave lane, even if you're a premade) and then drop your ward quickly and return to lane. While you're warding, your eyes should be divided between the minimap and your lane, so that you can figure out if you need to turn around early and rescue your carry.