If that is the case, then yes, go with how you usually buy On's shoes. The only time I wish I bought a different size from On was with their Cloudultra because it felt tighter than normal.
they stay in place once you set them to your feet. You never have to tie them since they rely on their elasticity to allow you slip your feet into the shoes. Once you have the shoes on, the laces don't really let go.
