Amazon and people who have been hit by this seem to disagree with you:
"Pricing is per instance-hour consumed for each instance, from the time an instance is launched until it is terminated or stopped. Each partial instance-hour consumed will be billed as a full hour."
"You are billed for an EC2 instance-hour for each hour or partial hour (rounded up) that your instance is in the “running” state. Instances that are in any other state (“stopped”, “pending”, etc.) are not billed."
Here's a company that got hit hard by that behavior :
"A little-discussed fact about AWS EC2 pricing is that users are billed for each server that runs for any partial hour it runs. That means if a user starts a server and then kills it within five minutes, he is still billed for the full hour. That seems acceptable, but if a user kills a server and replaces it with a new server of the exact same type and location, this move doubles the bill."
Instance hours are just rounded up.
Source: I auto scale my companies entire stack.