No snake on this planet NEEDS an undertank heater. Where this rumor got started idk. People like them because they are easy to deal with and usually very consistent in performance. Many breeders and keepers will just hold a room at a set temp that is acceptable. Monitor your snakes temps closely with a good thermometer and if you are comfortable then do that. Then continue to monitor the health of your snake. If it is eating and pooping fine don't worry about it.
There are many ways to skin a cat, and just the same there are many ways to get heat to the appropriate range for your snakes.