you need redundancy...not only should jack stands be deployed but the 2 lift jacks you shall require to lift the car up initially should be actuated and stationed strategicaly to 'back up' the floor jacks, just in case. I typically station my lift jacks around 6-12 inches from the floor jacks and bring them up until they are just barely touching your chosen jack point.....then lean on whichever end you're working and rock it back forth laterally and see if it moves....always worked for me