What do you mean it won't center like it should? Does it center, just off a bit to one side? Does it keep trying to find center? Can you use it in a game and it just slowly lose center over the course of a race? Is the optic wheel cracked? The washer trick helps if the crack is on the inner race of optic wheel. If the cracks are through the decoder windows, then it's not going to help much. I suggest getting a new optic wheel since those are easy enough to source. Spring for the brass one for best results.
Remember, when writing a post for help, being as specific as possible, covering as many variables as you can before posting and adding it to your post will help lead to a better answer. Hook up the wheel, make sure you have the logitech gaming software installed, and test your equipment using its interface. Does it react when you turn the wheel? When it does the auto calibration when the wheel first powers up, does it go lock to lock, and the when recentering, does it free wheel after (meaning, does it not stop at center, but rather just just keeping going till it comes to a stop naturally)? All of these help in troubleshooting wheels.