Yes, the BIOSes are set to the stock settings for the supported CPUs and the supported RAM timings. Another way to look at the RAM is that the high-speed RAM are all basically OCed to get the high-speed settings advertised. Hopefully, the reputable RAM mfrs. have tested the actual sticks they package to be stable at the specifications.
When you think about it, it makes sense because most people don't even want to OC their machines; they want to assemble them, run programs, and play games. Only in the past couple of years have the board, RAM, and CPU mfrs. even encouraged OCing (e.g., Intel's non-Extreme Edition CPU have locked mulitpliers - you have to buy a higher-speed (and higher-priced) stock chip to achieve the guaranteed performance.) With Board mfrs.' adding BIOS selections for FSB, BClk, voltages, and RAM timings, the door has opened to let the end-user access the settings needed to OC the CPU and the RAM.