I agree and know what your talking about. I made a tote to carry my quilting stuff and didn't write down any of the measurements as I worked. Since I took the basic concept from different ones I had seen online and mixed different elements together, I didn't really use a pattern. I had someone ask how I made it. Short of making a second I don't really know, I guessed my way through it.
So yes, it is a good idea to write all that stuff down as you go along. Makes for sharing later easier or just for finishing up easier. I need to work on writing down things to.