Watch some of the pros jungling. With the squishier junglers, you can't just stand there and facetank the new jungle mobs until you have a few items. Also, if you have to go back after 3 camps, that should be enough to upgrade your jungle item. That's what you really want, on any jungler, to be able to survive on those first 2 pots long enough to do 3 jungle camps. The jungle item upgrade gives you a big boost to health and mana regen as well as kill time, and Ranger's Trailblazer's stun and health/mana regain makes it MUCH easier for almost anyone to jungle. Mana regen doesn't do anything for Rengar, of course, but the rest of it is still super helpful.
Sure, some of the other jungle Smite items are sweet, too, but if you're struggling in the jungle and you're not getting Trailblazer, you're really just making your life harder.