If you use an optical cable, that means the receiver's DAC (digital-to-analog converter) will do the job instead of the one inside your CD player. If your receiver has a higher quality DAC, you may notice a difference.
Some people prefer to leave the digital-to-analog conversion until the last possible moment in the signal chain, which in your case would be inside the receiver. The reason for this is simple: an audio signal running along an analog cable is susceptible to interference which could degrade the sound quality, while a digital fiber optic cable is essentially free from data loss.
To be honest though, if you have decent enough analog cables, you may not even notice the difference. You can always try it out though - just return the fiber optic cable if you don't see a benefit!
