This cannot be generally answered. It depends on what changes have been made when rooted.
If you just rooted the device and installed some apps, but did not modify font, preload or dictionary partition, you should be able to unroot without trace via restoration of the system partition, boot loader, recovery partition and a factory reset.
If the device is obviously rooted, Sony will deny guaranty. If there are some hidden traces, they have to examine the device thoroughly, which I doubt they'll do.