You were probably using it for days with no problem because you had DMA enabled on your drives.
It only affects anything when the drive is in PIO mode (i.e. no DMA).
The whole problem with setting it to 66, though, is if your DMA controller ever reports an error, the kernel will reinitialize the IDE bus in PIO mode. Which means it'll start using the idebus=xx parameter. Which means you'll (very likely, according to what little I understand of the IDE system) start losing data. Obviously this hasn't happened to you, but I'd hate for it to happen to someone else.