The questions you need to be asking are.
Is there even a remote possibility that this widget can catch fire or electrocute someone?
If the answer is yes, (which it almost always is) then.
How big is my bank account? Can I afford, and find, the insurance? Can I handle living on the street for the rest of my life?
The thing people always forget is, UL provides a measure of rear-end-coverage. It indicates that you have done your due diligence and makes it a lot harder for someone to sue you when things go wrong.
It's also not about complexity so much as which components you include and how they are connected. For example a small led flasher with a AA alkaline battery probably doesn't warrant a UL approval. But if it has a AA Lion battery, capable of starting a fire, it probably does.
If you are on the fence, spend a few dollars and ask a lawyer.
NOTE There are also a lot of excellent comments attached to this answer. Read them carefully.