Unless you have high quality RAM, it doesn't matter, because if you have generic RAM then your PC probably won't even get into Windows if you change it.
Ok, it's not that bad, but if your RAM isn't the best quality, the system will probably be really unstable.
If you do have good RAM (Corsair, Kingston, Crucial, etc.), then lowering then a notch will boost performance by a considerably amount. Even with Kingston or Crucial, lowering it more than 1 notch can cause system instability. Simply enough, buying low-CAS latency RAM is better than having to lower it yourself, mainly because if it comes low at factory settings, then you have a performance boost without the instability.
You can try it, and if it works, your system will do better. If it doesn't, your system will freeze up, randomly reset, or simply not turn on at all.
If it doesn't turn on, then you need to reset the CMOS. There really isn't a way to perma-fuck your RAM; it just gets aggravating resetting the CMOS over and over.
-Lasereth