I really wouldn't worry about it. I'm on a 500 gig data plan, heavy app, email and twitter. I never go over. I always have more than 300 gigs left over every month. There are plenty of apps to give you a look at the data you use.
Sorry for my stupid question.
But as i know some apps work without BIS plan, so that means they would use the data but not through BIS?
besides, there are some BIS plan showed only included Local BlackBerry Email Data Usage.