Yeah I've done that since I got my first chinchilla. Although it's overkill since a weekly log is what breeders will use, it's a good way to keep track of health. Chinchillas usually don't start acting sick until they're almost dead, but they will often eat less when there is something wrong and this will show up in the trend. So I log his weight, water intake(measuring in cm), and food intake (giving it a grade of 0 to 5 and put it into my computer so I can see the trend lines. This helped immensely in getting a diagnosis for malocclusion in my first chin. Since my new chin is still growing the line is measuring his progress and it's fun to watch.
I recommend that everyone who owns exotic pets weighs their animals at least once a week. If you suspect they are sick but aren't sure you can sometimes confirm it by looking at the trend.
