I know that there are different opinions about
the "full bladder", evidenced by what members have posted in the past, so the first thing to do is confirm what you should be doing with the RO.
Be sure you understand - Are they saying "you don't need to" as if it would be nice, but not required? Or that they advise against it? Is there anything in your patient forms about it?
My CT scans, xrays, and all IGRT sessions were done with the full bladder.
My RO REQUIRED the full bladder for all prostate patients, and that was her specialty. You had to sign a form saying you understood the medical reasons, and had to initial in each day on the check in sheet. There were water coolers (covered with "have you had your water" signs) with the right sized cup just in case. If there was a delay for some reason, and you just couldn't hold on any longer, you peed, then drank another round and waited.
Even with all that, I have some bladder neck and colo-rectal burns that emerged as issues 18 months after treatment.