I have 8 Background Tasks, 3 of which are internal (don't need the Internet) so only affect battery, not data. I have another 4 or 5 apps that use "push" for alerts and updates (BGTs pull data). And 6 email account that use "push" or timed intervals. Data and WiFi are on 24/7. I average 225MB/month of data, most of which is attributed to background activities (WiFi goes to sleep when the phone goes to sleep). So about 7.5MB/day. I also burn through about 5-6% of my battery per hour while the phone is asleep, with all the above running, depending on signal strength. Even with everything above disabled, drain would still be in the 2-3% range, so not that big a hit for what you get.
Unless you are planning a very limited plan, like 200-300MB/month, you should have no problems Just enjoy your phone and stop worrying.
Sent from my Lumia 900 using Board Express Pro