The rear ones actually. If your rear end slides out its a lot harder to recover than front end slippage. Many people mistakenly put the best tires on the front.
While I agree, some physics-challenged lawyer set a precident in the usa sometime in the 00s. Almost all shops say they have to. However, if both are decent, its prudent on fwd cars to rotate them as they wear. The simple lesson here is buy quality tires, and replace them when they are worn, and many other issues are eliminated.
For a quick explanation to those unititiated in auto physics: bald tires on front = increases understeer. Bald tires on rear = increases oversteer. Understeer = car pushes straight under braking, going wide in turn. Default setup of 99% of road cars and considered safer in general for average drivers. Oversteer = rear end of car comes around, usually resulting in a spin, spins remove all control from driver. Alert drivers in well handling cars can countersteer into the slide sometimes, this is (basically) drifting. Generally, only sports and racecars are setup this way.
All of them need to be up to par.