Leak down tests the upper 2 rings pressure sealing ability, valve to seat interface, and head gasket...
A leak down test will not identify bad oil control rings or leaking valve stem seals.
A leak down test will let you know if you need to go back in.
I would expect a "freshly" built engine (if done correctly) to have less than 5% leakage.
I'd prefer to see less than 2%
Anything over 10% and I'd be pissed.

I have finished the build on the 2UZ I mentioned earlier.
It's going back into the Land Cruiser this weekend.
On the stand, cold, on a used 270K short block (nothing done to it at all) I hit 90-92% retention on all eight.
I was going to wait till I had hot comp test and leak downs to post but I thought you might want numbers on a "good used engine" tonight.
All of the leakage was passing the rings, Much easier to hear that with the pan off.

Oil consumption will usually make both leak down tests and compression tests a little tiny bit better because everything is "juicy" with oil and lubricating any seal helps.

From what I have read, the general rule for valve guide seals is that you get smoke when giving it power after a while of operating at high vacuum. If it smoke continually then it's probably something else.

--Ian

Codrus is right +1

Skylinecalvin,
I'd comp test and leak down test what you have now to see if internal surgery is necessary.
If the tests came back good I'd go drive it in a sporting manner.
If you notice the oil in "puffs" especially directly after down shifting and decelerating using the engine. (high vacuum) I'd change the stem seals first...
This can be done in car without taking head off.

949/Supermiata is the place to get your valve stem seals from.
They sent Mahle stuff to me and the package said "made in Japan"
Made me do the "happy dance".

If you can see oil smoke (grey/blue and lingers) during cruise then rings are most likely culprit.
Normally if you have oil consumption trouble with rings the sealing of the upper 2 rings is also poor (poor machining) and it shows up in a leak down test.

It is also possible to have oil consumption via the rings with low tension rings (like the NPR rings the SuperTecs come with) but the visible smoking is minimal.
You just notice oil consumption via oil level on stick.