They're not a must. That being said, it depends on your shoes and how often you go on ice fishing trips. If you don't feel like your shoes have sufficient grip, feel free to grab some traction cleats - that'll at least make you feel safer. And if you go on ice fishing trips more than once in a blue moon, ice cleats would be a good long-term investment anyway. As they don't cost a fortune, it won't hurt to have some additional grip.