Is it wrong leaving Wi-Fi enabled on my iPhone 4s when I'm home?

Does that decrease battery life or damage it in any way? Or should I just set the Wi-Fi to "off" and enable it only when I need it?

It decreases battery life, as in needing to charge it. But it doesn't hurt the over all life of the battery. The thing about turning it off though too… Is that you run the risk of data going through cellular instead of Wi-Fi. Which would add to the data charges.

No, only when your not home, this is because the iPhone 4S will continuously search for a signal, draining the battery.

I would leave it on, then if leaving the house, turn it off.

Wi-Fi drains battery, but this won't effect the battery life
whenever you don't the Wi-Fi, turn it off