Because the amount of traffic caused by live tile updates differs from app to app, just check the numbers in the task manager:
In my personal experience, the traffic caused by live tiles is negligible. If you're using apps that show images in their live tiles and those images change frequently, those tiles might cause a significant amount of traffic. You can keep an eye on the traffic caused by live tile updates in the task manager and disable apps that download too much content. Just right click on the app and select Turn live tile off
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Whether you need to disable the live tiles for an app totally depends on your traffic plan and the advantage you gain from the tiles. For example, you certainly want to disable an app that wastes 100 MB of your traffic budget per month just for live tiles.