It will depend on what you want to get out of a particular brew. Some styles require a drier, lighter body, and others will benefit from a sweeter, fuller body. Most of it will be experimentation, and finding what works for you. Make a beer, and split the mash in two, one high temp, one low temp, and see how different they can be.
If I want a drier light bodied beer, I mash at 148*, and If i want a fuller bodied beer, I will mash at 156*.
Most of my beers fall in the middle, at 152-153*. My Scottish Wee Heavy is mashed at 160*.