From the time you were a teenager, someone in your life taught you that you have enough tire tread if you can insert a penny in the tread and it reaches close to Lincoln's beard (if the penny is upside-up). Okay, well, that may work great on new tires from tire dealers, but it should not be the standard all the time. Here are some valid reasons why.
Not All Tires Wear the Same
The reason why your tire sales associate tells you to rotate your tires is that none of the tires wear equally. If you measure the tread on one tire with the penny test, you might measure the tire that has more tread than all of the others. Then what? You end up driving on the other tires that may be nearly bald. Conversely, you could test the nearly bald tire, and end up buying new tires when the others may have enough tread.
Tires Flatten and Blow out for Other Reasons
If the purpose of testing your tire tread is so that you do not get a blowout on the road, you should keep in mind that other things cause the tires to blow out. Changes in temperature and barometric pressure can flatten your tires too. When tires flatten as a result of temperature and/or barometric pressure, the penny test is also affected. Tread sinks away from the penny when the tires are leaking air, causing the penny test to fail the tire. When you reinflate that tire, the penny will register a thicker tread.
Different Standards Are Used on Used Tires and Spare Tires
Used tires are safe tires in good condition. They often just pass the penny test, but they are ruled by a different standard, too. Additionally, those little spare tires (often called "donuts" because of their smaller size) are also held to a different standard. That does not mean that these tires should never be used, nor should you avoid buying them. It just means that the penny test does not really apply.
Instead, you should look for bald spots on these tires. There will be places on these tires where there is no visible tread at all. The bald spots are interspersed with high tread spots when the tires are ready to be replaced. You can buy and use used tires for a short time, or until you are able to purchase new tires.