Yes Kai, it is best to have the shortest connection to the motor. Unfortunately, many people who use these variable speed drives fail to read the instructions carefully. Many people have found out the hard way about the emissions these drives have - they generate three phase pwm (at about 4Khz) and rely on the inductance of the motor to smooth things out. This is all fine as long as you keep the wiring to the motor shielded. There is cable made specially for this purpose or use steel conduit (make sure everything is earthed though). The makers of the 'special' cable also claim that because their conductors are equally spaced that this helps to eliminate standing waves. As to the truth of this, I'm not sure.

As for the internal circuitry of the variable speed drives, usually there is a small switcher to power the microcontroller and drive circuitry and the drive to the IGBTs is usually via transformers or optos. The RS485 is normally not opto coupled. Due to the isolation from the various transformers etc, there is probably capcitive coupling and thus using the correct 0V wire will help things enormously.