If you are going to be doing any work underneath you car you should make sure that it is properly supported before crawling under it. Often time’s people will use what ever they find handy to prop up the car thinking that it will keep them safe, but cement blocks have been known to crack and crumble under the weight of a car, and a stack of rims can slip and collapse letting the car fall.
It is easy now days to find safe and affordable axle stands which will hold the weight of the car allowing you to work under it in safety and comfort. Just use the jack to raise the car up and then place the axle stands under the car so that you can take some of the weight off of the jack and let the axle stands support the car. Most good axle stands will have either three or four legs to give it a stable base and a y shaped top that allows it to hold the axle or frame with out letting it slip. When the car is held up with the axle stands it should be solid and not wobbly, allowing you to work under the car safely.
Some people may say that if the jack is strong enough to jack up the car then it should be able to hold it, but if you stop and think about it the jack that they are talking about probable came with the car which means that if it is an older car it may be a bumper jack, or if a newer car maybe a scissor jack both of which are only made to raise the car high enough to change a tire, and then they are not very stable. Even a mechanics floor jack can have a seal break and let the car down in a hurry. So to be safe while working under your car be sure to get a good pair of axle stands, and use them, after all we want to be safe and not end up with damaged cars.