I have a Tubus Cargo rear rack. I bungee my sleeping bag and tent across it transversely. It works fine. I have a Jandd Extreme front rack. It's wide and has a platform. I haven't noticed a handling problem.

I think the centralization of mass principle is a lot more important for a motorcyclist than a touring bicyclist. The principle may hold up, but I'd guess the effects are trivial. But I'm no engineer.