I personally favor shooting groups. There are many that will advise just one arrow. My thinking is that as you shoot multiple arrows, you begin to see a pattern in your shooting. That pattern is a great way to determine if you are shooting the same way each time. When you just shoot one arrow, you may or may not hit where you want but you will be less able to determine what in your shot caused a hit or a miss.
If for example, you are shooting 5 arrow groups, and 4 of them are tight and one is a "flier" you get instant feedback and can determine what may have changed. But if you are shooting just one arrow, you have no immediate shot "feeling" to compare too.
Archery is partly a sport of consistency. If you are only shooting one arrow, and never repeating a shot, how can you tell if you are improving your form consistency or just reinforcing a form that may change from shot to shot?
Arne