It is not necessary to use a ring with the 100mm macro, no. At least not in the same sense that's it's necessary with the very long and heavy fast telephoto lenses that would be difficult to balance and would put a great, possibly fatal, strain on the camera's lens mount otherwise. You can use the lens safely and well with the lens mounted to a camera that is mounted to a tripod.

It's still something worth doing, however. If you are taking pictures of something off in the distance, the couple of inches (or handful of centimetres) difference in point of view you get when going from a horizontal to a vertical orientation (landscape to portrait) don't make a whole lot of difference most of the time. Flipping your tripod head to vertical while shooting macro subjects, though, will probably mean that you need to adjust both the position and height of your tripod. With the lens mounting ring, you just turn the camera and the lens stays pointed at the same spot. That can save a lot of time and frustration, and lessens the chance that a lively subject will leave while you're setting up.

It's also a lot easier to use a ring on a typical monopod, which will often have a tilt-only head if it has a head at all (some are just 1/4"-20 screw on a stick).