Indeed, Grace's Ice & Water Shield or a similar product must be applied at the roof edge, going at least to a point two feet past where the wall is underneath. The rolls are three feet wide, so that a single roll will suffice for an overhang up to a foot past the walls. A wider overhang will require a second roll. The material also should be applied over valleys in the roof surface.
Whether or not to apply the material over the whole roof requires some thought. It may be appropriate for a roof with very low pitch. For a more normal roof pitch of, say, 5/12 or steeper, total coverage is not required. There also is the matter of permeability; the stuff is a complete vapor barrier, meaning the roof deck will not dry to the outside and thus must be able to dry to the interior. This is not a problem if there is a vented attic space underneath, hopefully with continuous ridge and soffit vents. If you have a cathedral ceiling below, with vapor retarder and insulation applied directly to the rafters, then you really should not have Ice & Water Shield applied to the whole roof surface.
If you don't have ridge and soffit venting, a reroof is a good time to have such venting added. Simple gable end vents really don't do an adequate job of venting. If you have gone into the attic in winter and found that there is a lot of dampness under the roof deck, this tells you two things. First, there is too much leakage of air (and humidity) from the house into the attic space, costing you a lot of heat and creating conditions favorable to rot and mold on the roof deck. Losing heat to the attic also can result in a lot of ice dam formation and the damage that can cause. Second, the moisture that gets into the attic is not being vented properly. Continuous ridge and soffit venting takes care of this.
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