Dorans ring is an AP item, is he a AP guy then? So when you say don't waste it on a low hp enemy you basically mean let him die naturally and use it on another person?
AP isn't bad on Warrick at all since it makes your Q spell a LOT better. Also all the time you have mana you can heal yourself much more effectively then with a dorans sheild.
I've recently started jungling but it's very dependent on your team working with you. If people push their lanes it makes it very hard for you to get kills.