Better to have none, save money and keep the clean looks of your bike, in my opinion. It's so easy to check your system is charging or not. Just hold your hand in front of the headlight and rev a little.
Old British bikes used to have ammeters to monitor the frequent 'strikes' of Lucas. I believe you still find them on the Enfield that - not 'Royal' anymore - is built in India nowadays. Later on the automotive industry moved to voltmeters which give a better (not much) indication of the battery condition.
Warning: all extra gauges can distract riders attention, the more dangerous whilst balancing on two wheels and steering by moving your body! (Gauges mounted somewhere below on the engine are definitely a no no, imo.)
I pleaded the development of an electronic engine guard system in this forum years ago. So far nobody took the challenge to develop a little, easy to hide system that monitors temp, pressure, voltage etc.
For those interested:
forums.sohc4.net/index.php?topic=44306.0