Well what do you mean by it will charge the reserved instance amount? Do you mean by that just the hourly reserved costs, or the up-front payment that varies based on the instance period or instance type? If the former, then I stand corrected!
From: http://aws.amazon.com/ec2/reserved-instances/
Easy to Use: Reserved Instances are easy to use and require no change to how you use EC2. When computing your bill, our system will automatically apply Reserved Instance rates first to minimize your costs. An instance hour will only be charged at the On-Demand rate when your total quantity of instances running that hour exceeds the number of applicable Reserved Instances you own.