Depends what you are trying to analyze. If it is just your general state of ketosis, then the morning is a good test as you are generally lowest in blood ketones in the morning and ketones rise after exercise or eat a good keto meal. So if you are in ketosis in the morning (over 0.5 mmol) then it is a good bet you are in a good state of ketosis all day. You can also test before and after a meal to see how a food effects your state of ketosis.