I have most of my components in a single component rack. The bad part is that the cables coming out the back can be in close proximity to each other. I have tried to separate the power/interconnect/speaker wires/cables as much as possible. What are the rules to make sure to have the least noise and best sound? I have heard that when you cross cables, do so at 90* to each other, any truth to this?

I've not heard that, but it makes sense from the standpoint of not wanting a power cable to run directly alongside an interconnect for any distance - crossing at 90 degrees would minimize the exposure...

i have not heard that either. separating your power cables from your ics and speaker wire is about as good as one can hope for.

The 90 degree rule is accepted practice. It generally refers to power cables that must cross line level audio cables. Crossing them at 90 degrees limits the exposure to a bare minimum, if there must be exposure at all.

Designing and wiring for lowest noise is a whole topic in itself and books have been written about it.