I am a nurse in a hospital NICU. We do these tests on any baby who has had a UTI from urine obtained by an in and out catheter. A bagged urine specimen could be contaminated. A cathed specimen, if obtained correctly, shouldn't have contamination. The reason you want to have these tests is to rule out reflux into the kidneys. If urine is backing up from the bladder into the kidneys, it could cause a life-time of problems. Generally, I would say about 1/2 of our kids who have a UTI end up with this reflux problem. They are put on a low dose preventative antibiotic to prevent further infections and damage to the kidneys. You can discuss with your doctor an option to wait until the next bladder infection. But as one poster said, you could have a lot of damage done by then. Usual tests we do are an ultrasound of the kidneys, and then a VCUG.
Good luck. You have to do what you feel is right, and what you can sleep with. I will tell you, when my son was 4, I felt a lump on his abdomen after a small accident. I took him to the ER to get it checked out. The lump was located over his liver. I had a PA tell me it was his diaghram. They took an x-ray, and saw a mass, but my ped, who I just spoke to on the phone wondered if it was stool in his intestine. (She had not seen the x-ray, so was speaking off of the top of her head.) Their CT machine was broke, so they asked me to return in the am. The next morning, I couldn't even feel the lump anymore. But because I wanted peace of mind, and didn't want a problem later, I took him in. Turned out there was a tumor on his liver. 24 hours later, he was in surgery having it removed, and the surgeon prepared us for the worst, Cancer with mets to the lung. Fortunately, we got lucky, and it was a benign thing, rare, and it would have needed to be removed eventually because the liver had started to make room for it. My point is, even if something looks OK, you don't know what's happening under the skin. Peace of mind is wonderful, and knowing you aren't missing something that could have been picked up earlier is great. However, if you find out everything was normal, and will be kicking yourself that you put your child through this unnecessary test, than maybe skipping it is what is best for you. I knew I wanted peace of mind.
Good luck in your decision. Doctors and parents should be partners in their child's care.