If you follow the guide, you should be able to root without Unlocking Bootloader (Which will cause you loosing TA if you Unlock bootloader first)

So, the best way is to Root (Without unlock bootloader) which is in the guidance, then Backup TA, then you can unlock bootloader if you like (to install ROM or other tweaks).

For myself i just need root so i kept the Bootloader lock and have a backup TA just in case.