If you are above 70k miles or if it has been 50k miles since the last time your seals were replaced, I would recommend replacing them proactively. Seals are cheap and labor is expensive, and it doesn't really cost anything significantly extra to throw a few seals in while they have it apart. I see a large number of EJ22 and EJ25 motors with leaking seals in the 70k-90k mileage range. The SVX is not immune to this malady, my seals are leaking as we speak..

I would recommend replacing the tensioner, I have seen many of them go on Legacy and Impreza motors and the design is essentially the same. As they begin to wear and fail they make a noise that is easily mistaken for rod knock. I have also seen a number of them lose compression, causing the belt to begin walking.

As for the idlers, it is only common practice to replace them if they exhibit some type of wear or free play. If your shop doesn't charge an arm and a leg it might be worth doing merely as a "peace of mind" repair.