Does iMessage use data if you're on Wi-Fi?

I use the Wi-Fi at my house, and I want to know if iMessage uses your data when you're on Wi-Fi. I use my cell phone number, not my email address.
I think it does, because I use my email for iMessage on my ipod, and it uses Wi-Fi. But does the iPhone use its data when it can use Wi-Fi?

No it doesn't.

Messages are so littel anyways it doesn't matter… Unless you are attaching huge pics to every single msg.