Stephen, I'm afraid you have it backwards. To prevent ice dams, you need a cold attic. If the roof temperature stays the same temperature from peak to eave, ice dams won't form. Ice dams form when the roof over the attic is above freezing, and the roof over the soffit is below freezing. This usually happens at night.
Adding insulation to the soffit will do nothing to prevent dams, because there's no heat source to warm the roof. But adding insulation and ventilation to the attic will prevent snow from melting when the outside temperature is below freezing.
I never heard of ice dams while living in the Texas panhandle. And down in SE Michigan we have it easy compared to folks in the Upper Peninsula.