Cracked Screen Warranty Limitations?

Hey, new to forum but ive been reading for a while. i have both the htc one m7 and an EVO, now the EVO doesnt run things quite as well despite its really customizable. When i presented my rooted EVO they decided to just let bygones be bygones with the cracked screen. I have the M7 and im curious as if there is and where any SOLID PRINT, not theories or "well when i..." i want to know, if I root my phone and it bricks or defects due to an OTA, can i get a sprint store rep to help me? And if my phone is all tricked out, and gets cracked are there any papers i can print and proof as to how they are required to assist me? I will root the **** out of my phone when i get an M8 but i want to know for preventory reasons.