It's perfectly safe if you use a couple jack stands in conjunction with any old jack. Hell, I've used the spare one to get my heavy duty jack under there but on an even surface with 2 or more jack stands would be perfectly safe with just the spare as long as you don't do any heavy wrenching.

 

It isn't my first choice, but I do when necessary. Never had any problems. I usually use a lightweight aluminum floor jack with a hockey puck.