will tmobile know if i root or install a rom?

can you be more specific? In general or are you going to take it to the store/send it in ?
 
im keeping it but if the time comes i need to send it in , will it be already known to them?
 
As already stated they won't know if you un-root, return to stock Recovery and reset the trip counter before you send it in.
 
Well for the sprint store I go to most of the staff there has rooted phones(topic came up on a recent visit).

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