My Findings
I downloaded a few apps that track data usage and found out a few things. First of all, the Podcast app is the major data sucker. I shut it off, but it was still downloading all the time for some reason...and I never listen to podcasts anyway, so not sure why that is. I deleted the Podcast app completely, and data use went WAY down.
Second thing I found was that there is definitely a problem with the phone staying on Wifi when you have it off the charger. It sucks data all the time for various things while the phone is in your pocket. Biggest culprit there is Safari. I shut off all the Safari related operations and deleted all history, bookmarks, etc. and that stemmed the tide. I have gotten into the habit of leaving only one page open in Safari or a blank page, and that seems to cut down on the data it draws. I'm guessing that web pages left open that are more dynamic may draw updates.
The last thing to watch is to make certain that the phone is on the charger while you are sleeping. In the middle of the night, Apple likes to do its updates of their software and it can be a couple of MB of data that you are going to be paying for unless you keep it on the charger so the phone stays connected to the wifi.
Hope this helps.