When I started to learn piano about 2 years ago, I practiced scales with my piano teacher. For practicing C Major, she recommended repeating the tonic:
C D E F G A B C C B A G F E D C
When I switched to another teacher, he said I should not repeat the tonic and practice the scale like this:
C D E F G A B C B A G F E D C
Both of them were certain that their way was the right way to do it.
How would you advise a beginner (or even an expert) to practice a scale?