Jackstands are a must, and leaving the jack in place is a darn good idea. I am paranoid about getting the car on top of me and get slightly claustrophobic underneath a vehicle, so for added piece of mind I bought a bunch of 4x6"s (really not that expensive) and I stack those lengthwise underneath the rocker panels until they almost touch. That way even an earth quake can't bring the car down on me. Problem is your only point of entry underneath the car is either the front or back, but that's OK to feel safe. :-)

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)