If you ask me, it depends on the type of work you do. For my bows and other hair accessories, i keep a binder with photographs of each style, with information including the date i started selling, if they sold out, how many i made, what materials i used, how much of each material i used, the description I used in my shop, how long it takes me to make, how much i sold it for, and if i can get more materials. It's been helpful to me, i can make sure i'm not making a ton of things that sold slowly just because i finally sold the last one, and i know where to get new supplies.