At first I didn't use jack stands. After I read what can happen with jacks I won't go under a car without them at least in place to catch the car if it falls. Jacks have 3 problems.
They can tip, especially if you jack from the center to raise both sides at once. They can tip if your assistant trips over the handle.
They can collapse suddenly. It's just a tiny valve that is holding it up.
They can let themselves down so slowly that you don't notice until there's no room left to escape, no problem if someone is available to jack it back up.
Unless there's a good reason not to I leave the jack in place once the car is on stands. You can move things, get the car off the stands with less work, and it's a 3rd leg in case one of the other two fail.