T-Mobile doesn't know you are tethering. They can only see you are using data. They don't know if you are streaming a youtube video on your phone or browsing the internet on your computer while tethering. I'm pretty sure the issue is elsewhere. They will be able to see how much data you use (I believe your phone doesn't use that much). What really eats up data is streaming videos or downloading files (music, the latest ROM, or whatever). I have used the crap out of tethering and never had T-Mobile cut me off. I hear once you use 10 GB of data they severly slow your connection, but it should still work.
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