The more you repeat the basic lessons the better prepared you will be for the steps to come. Different students learn in different ways. I recommend you gauge your level of enjoyment as the deciding factor whether to memorize a lesson or move forward. The more you memorize the better prepared you will be for the lessons to come.
If you you're enjoyment level ever drops below a 7 (on a scale of 1 till 10) I would recommend you move to a new lesson, or go back to an older one.
You are, and will always be, your own best teacher. And the way you enact being your own best teacher is to trust your personal experience. Your personal experience will always be valid for you...and this will be what enables you develop a healthy relationship with yourself as you express yourself through the harmonica.
I personally believe that the more grounded you are in the basics, and the more fun you're having, the more likely you are to fall in love with the harmonica and play it for life (although I realize that sometimes people have more fun by not over challenging themselves). If you are an advanced musician, you may be able to play through a particular lesson once and be done with it. I leave it up to you to find whatever method works best for you.