Jacks can slip, slide out, leak down, or the user could inadvertantly release pressure.

If you jack up the car then lower it onto stands under the control arm the weight of the vehicle keeps the springs compressed. The only things I can think of that might knock it off are being run into by something heavy like another vehicle, or a BIG earthquake, and depending where you live the latter may not be an issue.

Jackstands under the control arms are inherently much safer and well worth the extra minute of work.

Duke