I would not be with out one. With out one your loads are an educated guess. Educated by published information and yours and others experiences. but until you know for sure how a load performs in your guns. its still just a guess.
I have found in my Guns Lymans published loads are the most accurate. Hornady's published loads are at least 100 fps less than I get.
Are they mandatory????? no. But real nice to know what is really happening.