In some cases, leaving your resources running maximizes your free tier benefits. For example, if you run an Amazon EC2 instance for only a portion of an hour, AWS counts that as an entire hour. Therefore, if you stop and start an Amazon EC2 instance three times in a single hour, you use up three hours of your monthly allotment…
http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/free-tier-limits.html