Has anyone experienced interference through harmonics? Is this a real problem or just something some "expert" used to scared people with?
Antennas for transmitting and receiving are designed around the wavelength to be transmitted. You can google the formula for antenna length. An optimal antenna is one that is exactly the length of one cycle. From there cutting the wave form in half produces the next most efficient length, and half again, and again until it doesn't really matter. So an antenna designed for 400MHz will also pick up 200MHz very well, and 800MHz just as well as the 400MHz signal. But it will also pickup just about every other frequency out there, just to a lesser degree based on the wavelength and transmitting power/location.
That being said, every unshielded conductor with current running through it is a transmitting antenna. Every unshielded conductor is a receiving antenna. This is the reason that when you create a PCB you should have the ground trace cover as much surface area of one side of the board as possible, so that it can provide some shielding to the components/leads on the other side.
I ran into major "noise" issues with my home automation system when I setup the house microphones. The top of the line sound card I was using for input to the computer was noisier than the cheapy sound card which was noisier than the even cheaper sound card built on to the mother board. The reason was that the higher end sound card had more traces and components, so picked up more noise from both inside the case as well as external. The motherboard based sound card has all the traces in parallel with the noise generating internal computer components, so picked up less RF from the computer itself, plus it lays parallel to the grounded computer case that absorbed some of the external noise as well.
This is also the reason why network ethernet cable leads have a specific pin out, each wire bleeds over to the next so the pin out is designed with a specific wire twist in mind to have unused (grounded) wires between used wires.