[imnuts]

No one "cares" that you flash custom ROMs to your phone, and no carrier can really tell that you have flashed a custom ROM to your phone. They may be able to tell based on you accessing their home page via IE, but they also have no way to snoop over the air as far as I'm aware to see if you've done anything to the device. However, as people said, you won't get support if you're having a problem with the device and you have a custom ROM on the phone, so if you need to get the phone fixed for some reason, you'll need to flash it back to stock. If you're getting your phone replaced, you also want to re-lock the HSPL. I also don't think any carrier encourages users to flash custom ROMs either. It would be far easier for them if no one flashed custom ROMs as then they'd know what every user had as a base at least and it could make support easier for them.