Nope, because then it'll eject it off iTunes thereby stopping your WiFi sync. When you disconnect it from the computer it'll still show in iTunes with the eject button.
No, it shouldn't need power - to enable Wifi sync for the first time, you have to plug it in & on iTunes check 'Sync over Wifi' (or something similar) and then your iPad should show up each time without having to connect the cable
A word of warning though....Don't perform wifi syncs. When your battery is below 50% or so....If you have a lot of stuff to copy over, you don't want the iPad or iPhone shutting down on you mid sync.