Have you guys verified this from your router, for example the Airport Utility's "connected clients" list? It will show wether your iPhone remains connected after it's been asleep for a while (or overnight).
In iOS 6 my 4S reconnected to WiFi so quickly that the delay could easily be masked by the slower wakeup animation in iOS 7 (when you press a button to wake your phone up it takes a while for it to react, followed by a slow fade-in animation).
Thats actually a brilliant suggestion i didn't think of.
I just refreshed my client list in my router, and its showing that only my MBP is connected. I haven't used my iphone in hours.
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Update: I used my phone for a few mins, and it popped back up in the client list again. Then after i locked it, a couple minutes later, disappeared from the list. So it doesnt seem to be making a connection.
But that doesn't mean its completely off or sleeping when your phone is not in use.
Regardless of signal strength, having both cellular and wifi radios on at the same time will drain the battery faster than if one or both of them was disabled. Add to that crappy signal strength and it will be even worse.
I will test this the next couple nights with wifi on and completely disabled overnight and see if there is any difference in battery drain.