The impedance should be 105 ohms in free air. Impedance is very important. This is the signal resistance a wire has under fluctuating currents. Since a round cable has an impedance of 75 ohms and a flat cable around 105 ohms, mixing these two can cause problems. In fact, impedance mismatches are one of the most common causes of SCSI bus problems.
Signal reflections tend to occur at sections of the bus where the impedance changes suddenly.
As you can see, the tolerances and specifications for the SCSI cables are tight. Therefore, only purchase SCSI cables from a reputable manufacturer. You may pay a little more up front, but it saves in the long run with fewer support problems. Never run the flat ribbon cable over or near the power supply. This has been a common practice in the past, but is not good.