Addendum
Another thing…
It may be tempting to conclude that the squirming action helps us out, because it helps tighten the crank without us stripping threads, etc. This would be similar to a procedure that was advocated a couple of years ago, which is to press the cranks on with a 20-ton press before the wheel is built.
However, this is deceptively incorrect. As Mr. Brandt points out, installing the cranks so that they are tighter simply leads to shorter crank life. Long-term stress resolution in the crank leads to cracks in the crank that do not appear upon installation.
If you install the cranks with the above procedure, you will not strip the threads, whether they are internal (crank bolt) or external (crank nut).
If you are following these instructions and using high-quality components, and still having troubles that replacing the cranks does not solve, then you should switch to a splined hub, where the interface is stronger, but the hub/crank choices are fewer and the cost is higher.
Note: these installation procedures do not apply to splined hubs in any way.