Not necessary, I ran without one for many years. But I did finally add one after all these years, just because I was tired of looking into the neck and wondering if it was losing coolant or not.
The only real problem is that the original cap was not designed to hold vacuum (so the water wouldn't get sucked back in as the radiator cooled). So I had the radiator shop change the neck to take a modern cap. Used a TR4-6 overflow bottle & bracket, mounted to the inner fender beside the radiator on the passenger side. Kind of a tight spot with the apron installed, but works well enough for me. Since the line from the radiator is all downhill, I can fill the tank (if necessary) from the radiator neck.
PS, I do seem to still be losing coolant very slowly. Not sure where it's going.