Cafeen, Here is a quote from Futaba's website.
Guidelines for setting up gasoline engine models. All ignition equipment, including an electronic kill switch, must be mounted at least 12", and preferably 14", away from all radio equipment, including throttle servos, etc. Ignition kill switch should always be on opposite side of fuselage from radio kill switch. All pushrods going to anything related to the engine must be non-conductive (just nonmetal clevises is not sufficient).
The Smart Fly switch provides more isolation and the other kind provides less. For $20 you can get more. A wise investment. Dan.