It really depends on the car... and I mean on which wheels is the traction.
Usually front wheel traction cars should have the better tires on the front wheels. But that doesn't mean that having good on front and highly used on the back makes it safe. The problem is that with usage, the technical caracteristics of the tire change and if a tire is tested to stop in 60 meters (for example) from 40-0 mph, a worn down tire will do it in 70 meters. This will cause an imbalance between the front or the rear. Fortunately many modern cars have ABS and the ABS sensor improves stability and safety even though there is a difference in the tires. From experience I do not recommend changing tires 2 by 2 because this will wear down the new tires faster than usual.
If your example is the Saab 9-3 (I had a convertible one :) ) then I recommend to have the least worn tires on the front because it's a front wheel traction car. If your car is a BMW, then having good tires on the back is more crucial. (Especially in winter time) Also it is very important to have winter tires in the winter, as the physical characteristics of the tire are different ("summer" rubber is made to be sticky, but at low temperatures it becomes hard, so if you have worned out summer tires on the front wheels and driving on the highway in a cold winter day, it is highly possible that you will turn the steering wheel and nothing will happen).