went through my owners manual this evening, highlights are rotate diagonally front to rear every 6,200 miles.
All 4 tyres to be the same brand model etc, for safe vehicle operation replace all 4 at the same time ( I believe this assumes rotation and equal wear) using tyres of non specified size detracts from controllabilitiy, ride comfort, braking performance, speedo accuracy and mile count acccuracy.
Then there is a warning box with the following in
'all four tyres must be the same in terms of manufacturer, brand (tread pattern) construction, degree of wear, speed symbol, load index and size. Mixing tyres of different types sizes or degrees of wear can result in damage to vehicles powertrain. Use of different types or sizes of tyres can also dangerously reduce controllability and braking preformance and can lead to an accident.'
So Subaru state it is something that can happen - a possibility perhaps but the thing is you will never be in a perfect scenario of equal grip on all four tyres at the same time for the life of the tyres so this is something you cannot reasonably counter other than the rotation of tyres but saying that dependant on your driving style you could have the rears down to the canvas in 3k and loads still left on the fronts.
I will always buy worn tyres in pairs and replace damaged individuals as they arise - ill let you know if something breaks but I wont be out there with a tread guage every two minutes not when the turbo clutch and brakes are more likely to fail before the powertrain :D