Never ever rely on a jack to hold up a car while working on the car, with the limited exception of an emergency roadside tire change. Always use jack stands and wheel chocks when working on the car. I would disagree with the use of wood blocks, with unknown weight bearing capacity, being used in lieu of properly positioned and rated jackstands. Best visual reminder is to get a tomato sauce can and drop it on a bug, then remember, you'll be the bug.