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crossovers can induce noise, as can things like wiring tied into your ignition system.. there are numerous ways for noise to get into a system, so it never hurts to be safe with the wiring.
I do have a few articles about power lines and RCAs though.
one is RC's and one is Buwalda's. Both claim it's a myth, but the shielding on the RCAs etc have a lot to do with it, too.
Their examples are ideal, whereas I can point out non ideal settings that pretty much negate all of their arguments.

System noise can be easily eliminated in 80% of all circumstances...install properly. There are a few cars out there that are going to be bound to induce system noise just because of factors out of the control of the installer and equipment. It is no myth either in the case of signal and power wires. Every power wire creates a magentic field around it, simple physics and the way electricity works. That magnetic field will change the characteristics of a cable/RCA's signal in the right circumstance, most cases that they are ran too close. it usually isn't a problem unless they are ran less than a few inches apart unless the wire is carrying massive current. The magnetic field around a wire isnt that large on a 12v system. There is no way around this fact though, I've done this exact thing in a DC electricity lab with my professor....it cannot be argued. Shileding may help, yes, but in 99% of all cases, it will induce noise.

Most problems in car audio are due to either the ground point, loop area of the RCA cables, location being near passive X-overs, or problems from the engine bay like a faulty diode pack, ignition noise, etc. Many people go buy the best shielded RCA cables they can get their hands on and end up disappointed as the foil used on these cables, originally intended for use in radio and high frequency applications, is designed to shunt out RF noise. In a car, this foil adds to the center conductor area and increases the ability for ground noise in the system. Common twisted pair unshielded cable is best.

I feel RCAs should be mounted away from power cable, not for preventing noise, but since MECP dictates this as you stated, and if you intend to compete, it's accepted as the right thing to do. But, the entire chassis of the vehicle is a ground plane and capable of inducing the same amount of noise, if not more due to distance, that you would get from a power wire and RCA beside one another, and since it uses the entire chassis it is irrespective of where the cables are located in relation to one another. Ideally you wouldn't have either to deal with, but seperating the RCA's and power cable is a moot point.

Then the name of your boss is Fred :) who drives a 300zx.

He posts on nolarides.com a lot.

Yeah why? How do you know that?

Do you happen to work for Superior Audio?

Maybe it does depend on personale preference if you do side work or something. But as an MECP certified professional installer it is proper to run the RCAs no closer than 18 inchs from the power wire. This is definently not a myth. It may be a myth to people who do this as a hobbie. But I know varies people who are at the very top in this industrie and it's not a myth.

It really depends on who you talk to on that subject. There are fairly reputable people out there that believe that its a total myth. I guess it all boils down to personal experience.

Check out the link below and scroll through and read Richard Clarks post. i have never personally tried this but who knows?

http://www.carsound.com/UBB/ultimatebb.php?ubb=get_topic;f=1;t=022690#000005

I'm an installer at a car audio and my boss drives a 300ZX. Well he was on a 300ZX fourm and he posted that he was taking the power wires out of his door for the old Bose stock system. He said that he was taking them out so that he wouldn't have the possibility of getting noise in the system when he ran his RCAs for his amp. Because you know that if you run your RCAs near a power wire to an amp on the highs that you could possibly get noise in the system because of the RCAs picking up the magnetic field from the power wire. This depends on the quality of the RCAs too. Well some dumbass posted back that he doesn't know what he is talking about and that running RCAs right next to power wire is perfectly ok and that noise in a system is definently a bad ground and has nothing to do with the power wire being ran right next to the RCAs, that that is just a myth. WHAT AN IDIOT!!! I think that if people don't know what they are talking about, they shouldn't say anything.