Any time you can. Powerheads increase the flow over the bio-bed. This increase is not only healthier for the tank, but also encourages a healthier environment for the de-nitrifying bacteria that keep your tank healthy. It increase the flow of oxygenated water over the bed.
Powerheads have been thought to only be used with ugf's, I also use them in this application. However, in recent years, some manufacturers have developed adaptors which allow the powerhead to become a powered sponge filter, re: Penguin 660r powerhead available at www.PetSolutions.com
. This powerhead also allows you to run a reverse flow application on your ugf's. I swear by this type of filtration. Reverse flow helps to reduce substrate compaction and reduces mulm build-up. However, I must stress that no system is 100% effective by itself. The addition of external/internal powerfilters and canister filters only improve overall filtration of the water. And if Toolman Tim had aquariums, he would say, "More is always better". You must take note, though, that some fish do not care for heavy water movement. Anabatids,ect, would benefit in the use of a reverse flow application.
Powerheads are also beneficial to create current. Fish that would enjoy this would be fish like gobies.