It depends on the type of %bf test. If you go with the 7 point skinfold method, then it doesn't really matter. This also gives you the most accurate measure of how much subcutaneous fat you have to lose, as compared to other methods that can't distinguish between sub q, visceral fat or hydration states.
If you go with another method, then just make sure you repeat it at exactly the same day of your cycle, same time of day, same fed or fasted state, same exercise state etc.... to make it as consisten as possible between different measurement days. This will give you the best measurement of true change in %bf even if the actual %bf is off.