The thing is, that method unlocks the bootloader. I am just wanting to do a soft root for access to Titanium Backup. Not really a desire as of now to run custom roms yet.
I am in the boat of waiting for the kind dev's to crack a new soft root method.
My method not only unlocks the boot loader, it also installs the custon recovery, so that you can install SU to be able to root the phone. Use my method, you do not have to install a custom Rom afterwards. and then you be able to use Titianium backup and and Nandroid backup.
for you the method would be,
1. unlock using HTC unlock.
2. install custom recovery
3. install SuperUser (SU) via the recovery
4. reboot
now your stock sprint rom is "hard" rooted. you are correct there is no "soft" root as of now. as of now, you must unlock the bootloader to to do any type of root. if you are concerned about warranty and support. its easy to relock your phone, just a single command, sprint will serivce locked and unlocked phones. they just need it to be locked to run the RUU they like to do when you have a software issue.