They aren't prone to disease, and they can't catch viruses. But a check up when the get older is always good so that you can find out if any health problems are there that you can't see. So about a check up once a year, or less if you take great care of them.
That's great that your parents are willing to pay. My parents are also, but I have a friend whose parent would refuse.
Me personally, I have never had to bring them in for other than a check up, except for recently, I got a baby that had URI already, but she is doing good thankfully.