I suppose that depends on how one defines "engineering". I consider engineering to be a way of formalising methodology to get consistent, repeatable, and verifiable results. My opinion on this topic is based both on this view of engineering as well as experience having worked at different times as an electrical engineer and software developer. I would say yes, mathematics is indispensable for good software engineering, although it certainly isn't the only requisite skill to meet the above definition (nor always the most important).