Well, no amount of ROM'ing and tweaking you do is going to put dust under your screen or otherwise spawn manufacturing defects in your phone. That would still be covered by whatever insurance coverage you have.
If you're still within your 30 day window, Sprint/your vendor (if they support that agreement) will let you return or exchange the phone no matter what you've done, as long as the
hardware is in new or near-new condition. Truly, they will either hard-flash and resell the phone, or ship it back to HTC to be refurbished (and likely sold again as new)
I'm of the opinion that when you purchase the phone, the phone should really be
yours. You pay the carrier (Sprint) for access to their network using
your device. What you do with it, as long as the act doesn't *directly* affect the service they provide (network access), should be your business.
It should really be no different than buying a computer at wally-world which just
happens to be subsidized by your local ISP (3 months 'free' with a new purchase!) - you'd still expect to have full, unrestricted
software control, but you might not want to, say, replace the motherboard and still expect it to be covered under warranty.
That said, if you happen to brick your phone and need an exchange, the first thing you tell the clerk probably shouldn't be
'So I was trying to root my phone and install this neato custom image..' 
