I’ve taught over 600 classes and I would say half of those were without using a microphone. As Elizabeth said above it really depends on the size of the class and the location/size of room. Also it comes down to what type of class you teach. If the instructor has to move a lot or breathes loudly this can often come out through the mic.
Microphones do wonders especially for the clients and instructor. When I have to teach classes without a mic I often feel like I’m shouting and my throat burns after. Yet there are problems with microphones such as some instructors don’t understand what is too loud which can damage hearing of clients. Microphones are also expensive which is one reason why I would use it at some classes and not at others.
All in all it comes down to how much you want to invest, if you want to play your music louder (can with a mic, can’t without), what type of classes you teach, and if you want to save your vocal cords or not.