Anything that can support a car, and often a thick chunk of ironbatk or redgum is going to be alot more stable than a made In china pos jackstand....do not be fooled into believing you are safe just because you bought jackstands,get the right jackstands..
As for wheels...do not support the whole weight of the car on them.no...but if you have a car on a jack and no stands available...At least throw the wheels or something under the car to prevent it falling on your head..
If a car falls on your chest you will die a slow painful death...all the air will be squeezed from your Lungs and you will be struggling to breathe so you can yell for help, but it will be impossible...you will slowly suffocate and it will be a miracle if you survive...so put whatever you can under that car to stop it...A thick block of wood will save you, A wheel will save you, a few telephone books could save you...bricks will crack and probably not save you..never use bricks !
Use common sense, Remember a car weighs between 1-2 tonnes and dont give it room to gain momentum...buy a good jack..that crappy emergency jack that's in your boot is just that, an emergency jack they dont last very long when used regularly....