You should always purchase as much insurance as you can afford. The lower the deductible, the more insurance coverage you have. So if you can afford a $100 deductible, it is recommended to purchase insurance with the lowest deductible.

The higher your deductible the less your insurance will cost. The lower your deductible the more your insurance will cost.

Remember, your deductible is what you agree to pay. Now, if you are talking about "comprehensive" and "collision" coverages, and you have a lienholder, that lienholder might dictate what deductible you are allowed to purchase.

Comment Update: The deductible is the amount you pay regardless of the value of the vehicle (at time of purchase or time of loss), if it is totaled, or if there is slight damage.