Definitely don't have to measure the food weight. I just prefer to so I can record and keep track exactly how much my BP eats. And since I shelled out money for a digital scale, then I might as well use it whenever possible. When I first started measuring mouse weights I was quite surprised by the difference in the true weights compared to what I estimated them to be. You may find the same, or maybe you estimate right on. Like previously mentioned, it's good to get experience learning exact weights, which will help you eyeball better in the future. There may come a point in time when you have many BPs and it isn't practical to weigh each rodent anymore.