I think it's a great idea to measure (accurately) beforehand and check periodically. Catch is, my sense from talking with a lot of folks about this is that not everyone experiences this the same way.
Lots of theories (about mechanics of this, lots of rules of thumb, but I've never heard of anyone doing a specific and objective study.
Part of it might be that folks just get used to walking in different types and sizes of shoe. I am (or at least was) nominally a size 10 or so, but ever since my first thru-hike I just always wear size 11.5 shoes, whether at home or on trail. It's not that my feet have grown to be "size 11.5" --- I think they have grown (permanently grown) a bit, but definitely not that much. I just like having that much space in my shoes now.