After reading through this posts, maybe I missed it but one should never, ever work on a vehicle that is supported on a jack or jackstands.
The blocks as described are the best, making sure that the load has been released from the jack and is bearing solely on the cribbing.
Another area that is often ignored is the substrate. While maybe not quite so worrysome in your garage, jacking loads outdoors often has its own challenges.
As was pointed out previous, make sure the fround you are working on is solid, preferably concrete. Soil, grass, gravel and even asphalt tend to give.
I have seen all too often where guys will lift a load with a jack and toss a block under the frame "just in case". Many times the load hovering above the block. This is foolhardy because if the jack slips, rolls, get nudged etc the "safety" block may fail to catch the load on the way down, get split, dislodged or otherwise be pushed aside.
The company I work for moves loads (mainly jack and slide) often in excess of 200-400t so I have a bit of an idea....
