I don't see what this has to do with overclocking or cooling.
But since I can't think of a place where it DOES belong... I guess I'll let it slide. Make sure the desk provides good cooling capabilities for maximum overclocking.
I kinda dislike armrests in general... And I keep my PC on top of a small drawer cabinet so it's away from all the floor dust and whatnot (that and I can roll it away from my desk to work on it easily). On the desk I just have the monitor, peripherals, and scanner. I also don't use my keyboard drawer (even though it'd be slightly more comfortable) as it places me too far away from the monitor imo.