Well I usually just run to each elite pack, sometimes dropping a hota if i got 20+ whites collected up and in the process of killing the elite pack any whites that kept up die along with it but there's prob about 40% of whites that I miss.
Technically, you have to kill enough stuff to summon the guardian and clean the rift to get the big loot drop, and the bonus XP back at town, but you could theoretically treat them as a quick source of bonus elite packs -- or assume that you will get enough white collateral damage finding and killing the elites to usually get the guardian out. So... maybe this is more efficient. Maybe.
Personally, I figure that you are better off just clearing stuff anyway, because more kills == more legendaries, no matter how you slice it, and I *bet* the developers tuned for "clears most white trash", not "clears only elites" with the drop rate. The less your play is like what they tuned for, the less likely you are to get that legendary ~ every two hours. Which might be good if they screwed up and elites are that much better, but I really don't think so.
have been in a rift with clan mate who just rush through every floor skipping white n kill only elite. end up reaching exit with 80+%, have to backtrack to kill white to summon guardian. efficient aren't we?
There are great rifts and there are bad rifts. Or more precisely: great and bad levels of a rift. A great level has an elite pack wherever you step, in those you can ignore whites and should kill all elites even after the guardian is out. Remember Warrior's Rest farming? This is better.
A good level at least has large amounts of white mobs like skeletons, killing them makes sense imo, because many mobs die per cast and therefore are worth stopping for.
Bad levels however present you with "packs" like 3 act2 dogs. Those suck big time. Should you skip them? Sure, very inefficient. However you might end up having to kill even such crap because the rift doesn't have enough mobs to spawn the guardian efficiently.
I've been at the end of rifts before < 100%
That's not great, sure, but I wouldn't clear bad levels unless I absolutely have to.