When I replaced my rack, I told the shop that I wasn't sure if my rack was centered, and they said they'd take care of that. However I to had to re-align my steering wheel. This job is really really easy, at leatst on my car, but i'm not %100 sure if it's the same for yours. All I had to do was disconnect my steering coupler that connects the steering column to the steering rack. Just make sure your wheels are pointing straight ahead before you disconnect the coupler. Once the wheels are straight, disconnect the coupler, and adjust your steering wheel so it's pointing straight as well (it helps a lot if you have a friend hold the steering in place while you reconnect the coupler). Also, you may need to loosen the bolts that hold your rack to the car, that way you have a little bit of room to play with while disconnecting and re-connecting the coupler.
***Whatever you do, don't make the mistake of putting the bolts back in the wrong way, I made that mistake because I was in a hurry, and I had to replace the coupler....I called Subaru for a replacement, and they quoted me over $180!!!
Here's what my steering coupler looks like....
