You might be required to if you care about having legal protection over your intellectual property. FWIW, I think most YC startups share their employee contrac

You might be required to if you care about having legal protection over your intellectual property. FWIW, I think most YC startups share their employee contracts, and in it, there's a section that mentions you are entitled to your own IP as long as you give notice of the side project AND you don't use any company resources on it (like working on company time, their laptops, etc.).