The twine stays relatively clean, I've actually never had to clean it.
Urates and stools become benign not too long after they start to dry. Your biggest enemy would be an overly moist cage. That could cause issues, but it would be quite odd to have an outbreak of something starting in the twine. I'd be far more concerned about a snake that soaks frequently in stagnate water.
Don't get me wrong, a clean cage is a good thing. That said, going overboard with sterilization isn't necessary. Your snake can build up its immune system by being exposed to lower levels of certain bacterias. Similar to humans, they can develop a tolerance, which is healthy. Again. I'm an advocate of a clean cage, but OCD maintenance is not as important as some think.
To get back on topic, twine will work well and isn't a big factor when it comes to sanitation.
Keep your snake's stress level low, keep the cage warm enough and monitor conditions to prevent extreme humidity and you can use about anything for perches.