I have a situation where I am creating features that I am intentionally not adding to my unique values symbology - I want them in my all other values category, but it turns out this is impossible to do. When creating features, you have to select one of your existing symbology values. There is no <null> option, meaning I have to just select one and edit it after the fact, which creates a lot of room for human error (which I am prone to!)

Is it a best practice to create data with intentional <null> values? No, but for my purposes, it's just me using this data, and it's much less work than creating another symbol class, domain code, etc.