If you switch off Cellular Data, your wifi can't revert back to it.
It's also true that you might miss some notifications while the phone is sleeping - but you'll get some too. I still can't find no rhyme and reason to this one. Most of the time though I'll get my notifs even if it sleeps.
And as Keith mentionned, your iPhone will choose WiFi over Cellular data when it finds a WLAN. It's also true that if both WiFi and CD are on while in a WiFi area, when it goes to sleep, you might see the 3G/LTE flash briefly when you turn it back on, before WiFi takes over. That doesn't mean it was using CD while it was asleep. It's just normal for the waking phone to "find it first" because the "phone" part is always communicating with your carrier's network. One thing though : when both are on, and your WiFi signal suddenly drops enough for the phone to switch over to CD, it might happen that it will stay on CD even though WiFi signal got back to full strenght. It's only when you'll go in Settings - WiFi to see what the hell is going on, that it will suddenly get back on by itself. That happened to me a lot when I was on iOS 5. And yes, my "Ask to join networks" was on.
I'm on WiFi 23,5 hours out of 24, on most days...