lagerlout pretty much said it. Its a good 10 years since I did electronics at uni, but basically any wires running alongside each other can induce a current in each other i.e. as if by magic, they can create current in the other wire. Thats how transformers work.
So if you ran a big power cable alongside an RCA pair which conduct a very low level signal, it is quite likely that the RCAs would pick up interference from the power cable because as Lagerlout said, its not direct current but is actually varying slightly. Not enough to knacker your amps, but enough to be picked up. This problem can be solved by running shielded RCA cable which most decent ones are anyway.
you could be lucky and get away with it. People probably dont bother, because its almost as much hassle to run cable down one side as it is to do both, so its not worth the bother of finding out youve got interference once the carpet it all back in again. If you do want to run the cables down the same side, make sure youre running shielded RCA cable, and try to keep them spaced apart. If youve got no choice about runnign them close, try to zig-zag the cables a bit so theyre not totally parallel.