The accepted standard for tire wear is when there is 2/32" of tread left or when there is unusual wear from misalignment. If you stick a penny in the tread, 2/32" comes to the top of Lincoln's head. The more the tread is worn away, the more likely the car will hydroplane on wet roads. If you live where you get a lot of rain, replacing the tires sooner is much better than causing a crash.
Some people recommend rotating the tires at regular intervals, typically every, ... Oh, ... 10,000 miles or so. That will make all four wear out at about he same time. I personally never rotate my tires because none of my vehicles are known for tire wear problems. The front ones wear out fastest, but then I only buy two new tires instead of four. Some vehicles, Ford trucks and small cars, in particular, have very bad tire wear problems and there's nothing you can do about it. Those are the ones where rotating very often is a good idea.