Your phone maintains a constant connection to Microsoft's servers for push email and to update live tiles. While email will update (as well as other live services; address book, calendar, messaging) the background, other updates will way till you turn your phone on.
For reference, I have data and WiFi on 24/7. Although I have an unlimited data plan, throughput over WiFi is much faster. With 4 email accounts (2 push, 1@30min, 1@60min) and 40-70 messages a day, 2 hours of browsing, about a dozen live tiles, Twitter, Facebook, occasional music or video steaming, and a bunch of other services turned on, I have consistently averaged 250MB of data per month. Before turning on WiFi, with almost the same settings, I was using about 2.2GB of data. So if you have a 200MB plan (a guess), with a little management you should be able to stay under the limit.
Sent from my HTC Surround using Board Express