Contrary to popular belief, gutters don't cause ice dams. While snow and ice can building up in your gutters, the root of the problem is heat loss through your roof.

I ran across this nice article by BCI Contractor which explains that by keeping your attic at the same temperature as the outside, you keep snow from melting on the roof and forming an ice dam around your gutters. Good insulation and proper ventilation helps avoid problems.

Worth a read.