To rake or not to rake snow off of roof
There are really two reasons to rake a roof (snow weight and ice dams), and there are two reasons not to rake (effort and roofing damage).
Raking to reduce snow weight
If you're concerned that the weight of accumulated snow will exceed load design limits, by all means pull some snow off. You don't want a collapse. This is a rare situation in northern climates, though, as snow loads are expected and accommodated in modern construction. Talk to your neighbors about how much snow is concerning. Take an average between the Chicken Little and the Couldn't Care Less types.
Raking to resolve or prevent ice dams
Whether you rake for ice dams depends on a number of factors, such as attic insulation and ventilation, sun exposure, and weather conditions. You'll have to make a judgement call on that for a given situation.
If you have an ice dam problem, removing the snow from the lower portion of the roof may help. By removing snow from the area that's melting you prevent dams from forming. If snow is melting due to inadequate insulation, and most of the heat lost from the home transfers through the roof down low, clearing that area may resolve the issue.
It may also relocate the problem, as has been suggested. Dams occur where ice has melted and re-frozen. If the snow is insulating the roof surface, allowing melting to occur due to heat from below, the dam may occur just below the new lower edge of the snow.
Raking is hard work and damages roofing
All that said, raking is hard work. You'll get a six pack. It also will damage your shingles if you're not extremely careful--those corners have a tendency to punch through the snow on occasion. Over time you'll scrape away the protective ceramic granules and weaken the shingles. They'll curl and crack before they otherwise would have. Raking is to be avoided unless truly necessary.