Originally Posted by fisherbln
If you place the jackstands at the correct locations, you should be fine. I push on the car in a few directions once I have it up on all 4 just to make sure. Honestly, the car usually feels more stable sitting on 4 than 2 (no danger of the car rolling off).
All that said, I am paranoid by nature so I actually use 6 jackstands in this situation -- the 4 main ones and then I use 2 "backup" stands strategically placed under the section I'm currently working under. Coupled with the jack itself sitting under a third spot fully ready to take on the load, I feel safe. I've been known to put my wheels under the car too just for additional coverage.
I was planning on doing all of that sand the 2 extra jack stands, though that sounds like a good idea. I'll pick up another set for myself before I jump under the car.
I've always felt safer with two wheels on the ground chocked.