Renaming the thread to "Get rooted for warranty service" since the procedure described in the OP applies beyond just broken screens and buttons.
Though I'll mention an interesting experience I had at my Sprint store... It was launch day for the Evo 3D. I already had one in hand from getting it early as a S|P member, and I simply brought it to the store to get it activated. The main reason I did it in store was because I recently cracked my Evo 4G camera lens cover on the back, and I wanted the TEP to cover the 4G. If I had activated the 3D, I would no longer have coverage on the old phone.
Anyway, I mentioned the cracked camera, and the rep looked at my account, saw I had the TEP, and said, oh we can fix that for you, no charge, and took my Evo 4G away from me. And it's running CM7. I got a bit worried about that, so I was like, "oh, let me shut it off first," and the rep said not to worry about it; the tech will need access to the phone to test the fix. Ok, so I guess they were going to find out about CM7
30 minutes later, the tech came out and said, "all set." He had replaced the entire back portion (the red part). He powered up the phone in front of me, and together we watched my custom splash screen load.... then the CM7 boot animation... Then he handed me the phone and asked me to test call someone. I did, confirmed the call went through, and he said I was good to go.
I would think that before the tech replaced the back board, he would have shut the phone down first, which means he already saw CM7 in action. He didn't come out of the repair shop making a fuss about the rooted ROM. Can't say that all repair techs would be so generous, but there's a good argument to be made that my damaged hardware had nothing to do with my modified firmware (root).
To be perfectly safe, unroot before you take your phone in for service. I may have gotten lucky. As of this writing, as long as you don't accept any OTA for 2.3, you should be able to re-root once the service is done (unless the service rep accepts an OTA on your behalf!).