You don't need it, but it is probably the most useful gauge if you are tuning yourself. It will let you know the exhaust temperature, so you can determine how rich/lean it is running and if you need to back off the timing because of detonation. I don't have one and I have gotten along fine.

If you want to go through the hassle of welding a probe into the manifold, it is worthwhile, but it isn't completely necessary.