75 ohms characteristic impedance is beneficial, but not necessary. If you want to minimize jitter, then stick with 75 ohms and get a 1.5m length minimum. Ask someone with digital and transmission line experience for advice. Read this article I wrote on S/PDIF for more info:
http://www.positive-feedback.com/Issue14/spdif.htm
http://www.positive-feedback.com/Issue14/spdif.htm