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.