In which types of situations is the smallest unit rate the most desirable?

You should use the smallest unit rate any time accuracy is needed. For example, if you are administering dosages of vaccines, you should use the smallest syringe possible for the dosage. A smaller syringe will have more precise graduations than a larger one. The same holds true for glassware such as graduated cylinders. A 1000 ml graduated cylinder will probably not have a 5 ml graduation on it, and measuring out 5 ml with a cylinder that large would be guesswork at best. A 10 ml graduated cylinder, however, will have graduations for every 0.1 ml, and would be suitable for measuring small amounts of liquids.