The short answer is - it depends. Racks center of 2 channel setups are more of a detriment the higher they are IME and even more so if a monitor / TV is on top. Also, keep in mind that front wall center treatment plays a more critical role the shallower and smaller a room is since the direct sound from speakers bounces off the rear wall and then again off the front wall to the sweet spot. However, the side wall location can also be problematic especially if the rack is placed at one of the first reflection points and further exacerbates sonic issues with less speaker toe - in.
The only way to know for sure as to what best meets your sonic needs in your room is to experiment and pick the best compromise.
Equipment Rack Between Speakers...Good or Bad
This question came up in another current thread and I thought it would be more appropriate to start a new thread to address it. My rack is between my speakers. In the past I have tried it off to the side and didn't notice any sonic advantage. I have seen in in the past that there are some strong feelings on this subject, and I am interested in hearing what everyone has to say.