I have had real problems with this in the past, to the point that I am a bit hesitant to buy loafers. I really like them, but I have had two experiences with loafers fitting well in the store and then loosening up over a couple of months worth of wear to the point that they began to slip on my heels. I started to develop a bad habit of curling my toes like I was subconsciously trying to "grip" the bottom of the shoe while I was walking. By the end of the work day my feet would ache because of it, and I learned later that it is not healthy for the feet to do this. I don't think that there is a universal rule, but the SA told me last time that I should make sure the shoes are on the border of being uncomfortably snug/tight when trying them on in the store. This may mean a half size down or more, depending on how they feel and the last type.