The only time when I can justify underclocking is when you're trying to fit within certain thermal constraints. This often happens in HTPCs that recieve a recycled CPU (like a x4 955) that is grossly overpowered for the type of work it will be expected to do and has a high TDP (125W in this example) that doesn't work well with tiny enclosures having little airflow.
Keep an eye on your gaming framerates in CPU-limited games when you're underclocking like this. AMD's per-clock performance is definetly worse than Intel which is why AMD CPUs are always clocked higher to achieve the same "Intel-equivalent" performance. (ie, an AMD FX-4100 at 3.6-3.8GHz games roughly as well as an Intel G630 clocked at 2.7GHz as shown in the following article) If you're just looking for gaming performance, tests have shown that many modern games are more dependent on per-core performance than number of cores.
If you're looking to save power and temps, try disabling one or two cores instead.
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod