Wow, bumping after 7 minutes....
Anyway, I would not under clock your ram.
Underclocking may get you slightly lower latency but it will also slow down the maximum throughput it can manage.
The ultimate test though is to try it. Run the ram at rated speed and test its throughput using software and then underclock it.
In my case, when encoding H264 1080p video I got 18fps with ram at 800MHz on my Q6600 and only 5fps when underclocked to 533MHz.