Stacking washers is never a good idea, ever. Back when I was in school for diesel mechanics they brought in two speakers relating to this topic. The first guy was a bolt engineer, all he does for a living is design and test bolts. The second was a washer and gasket guy, same deal there.
To condense the combined four hour lecture, putting a washer on a washer will cause bolt stress and eventual snapping of the head off the bolt. Another potential cause is unlevel wear into the surface between the washer and part being bolted down, this results in bolt head stress and eventual failure of the bolt.
Washers are sided, they should only be used in one direction, bolts should never be shaved, however in a pinch it can work. More important than that, you should never grind bolts above grade 5 or you ruin their clamping ability.
When installing bolts, you should always lube them. For young guys wiping it along the outside of your nose is sufficient, or you can wipe an oily hand or rag on the threads.
Torque wrenches are only accurate when moving, once they stop they do not measure accurate at all. When tightening bolts in critical applications, you should take all bolts to 60% spec then go back around to 100% spec on tightening torque.
You can use two washers, but it is never a good idea. The effects listed are usually less in automotive becuase of the lesser torque specs and much shorter lifespan, however bolt failure can occur in as soon as 80K miles if multiple washers are used in a static environment. In a dynamic envirnmet bolt failure can occur in less than 20K miles.
