@bgchz Placement in the center is acceptable, but generally the more free space you can have between and around the speakers, the better they will image. While there may be some that will say they move their gear away from between their speakers to reduce vibration, that vibration can also be reduced with better racks. IMO the real benefit is to allow the speakers, especially mids and high frequencies, to breathe with minimal reflection. This is also why many people opt for low racks, or simply place their amp(s) in the center with the rest of the gear out of the way.
But every room is different. Sometimes putting gear on the side wall will result in more vibration to the gear, so it could be a tradeoff!