Smartphones are data-hungry devices and you could run up huge bills (depending on which plan you're on and if you're abroad).
Modern phones often use data connections in the background for updates, syncing and other functions that you may not be aware of. All of these functions will affect your data allowance unless you connect via WiFi. If you regularly break-out of your monthly allocation and expect to continue doing so, your safest option is to upgrade to a plan that gives you a greater allowance of data then you currently receive for a fixed monthly price. In the meantime, please see the relevant section below for your type of phone.
You can find out a lot more about it and see some handy screen shots if you click here.