For maximum power, lock all the diffs with epoxy.
All other options exist to improve handling, not power distribution. That's why rock crawlers have fully locked drivetrains.
I have ~50k silicone oil in the center diff of my HPI Trophy Truggy, and it pops wheelies when I don't want it to. So you really don't need a locked drivetrain to avoid diffing-out with a truggy.
In my experience, the front and rear diff oil viscosities depend on ride height more than anything else. High-riding monster trucks need thick rear diff oil for good acceleration and thin front diff oil to minimize traction-rolling. At the opposite extreme, touring cars with <10mm ground clearance need thin rear diff oil to avoid uncontrollable oversteer and very thick front diff oil (or a locked front diff) for good acceleration. Most other types of RCs (buggies, truggies, etc.) fall somewhere between those two extremes. (drift cars and rock crawlers are special cases and thus have special drivetrain configurations.)