Are those slip knots or a stretchy material? As your plants grow those stems are going to get thicker. If those knots don't expand your going to choke the branches.
The leaves themselves can just get tucked, you don't need to tie them down. Your training the plant to alter the apical dominance which only effects the growth points on the stems. Auxin goes to the tallest growth points and caused it to grow so by tying everything down level auxin is distributed more uniformly.