Interesting comments...OW students do not know any better. While I am not the cold type, I will usually dive 60F or so water with my 7mm longjohn (no 7mm shorty) and a Bare Chicken vest (sleeveless hooded vest).
I personally think, it should be mandatory for the first dive of each day for a couple of reasons...one is to get used to properly adjust the mask with the hood ie, ensure the silicone skirt is underneath the edge of the hood not only when they are prepping up, doing buddy check but also when they do their mask clearing drills, don and doff underwaterl...Always better to do so in a controlled environment. Two, for them to realize that sound will be greatly reduce wearing a hood and you may not be able to hear a shaker for example. Three, the head is the area where people experience the greater amount of heat loss. Having somebody getting too cold on the first dive that they either cut it short or result in them becoming beach ornament for the rest of the day is counter productive for them and the instructor. If they want to experience it hoodless then do it on the second or last dive of the day, so they learn about their comfort level and adjust the remainder of their training accordingly.