Will 4g/3g use data just being on?

I have an iPhone with 3G and I tend to leave it (the 3Gon. However, I rarely actually use it to browse the internet, for example. I'm set up with the smallest data plan available (I don't remember exactly how many mbs I'm allowed per month). My question is whether having my 3G on, but not actually using it, will count towards my mb total for the month.

Some apps will update and sync in the background, so yes it will use some data.

Turn off the 3G when you're not using it, or use Wi-Fi.