iOS 5 automatically turns on the Wi-Fi to sync? OR Should I do that?

Hi.

I was wondering if I have to turn on the Wi-Fi and let it on throught the night when my iPhone is charging to make it automatically sync with iTunes or if the iPhone turns it on the Wi-Fi by itself....

I plugged it in to my dock waiting the iPhone to automatically turn on the Wi-Fi.. But that never happens. I have to do that to make the process work.

Anyone knows how this really works?