Probably. It's always best to get things like that in writing. You wouldn't want to have a situation where either:

1) The car breaks down in Alaska. When you call Roadside Assistance, Dollar says you violated the rental agreement by taking the car there, they won't come help you, and will put you on the Do Not Rent List.

or

2) Crossing from the lower 48 into Canada to get to Alaska, the border guard checks your rental contract for authorization to drive the car into Canada and beyond. They think the car is not allowed into Alaska, and prevent you from continuing your trip.