You may want to test sooner rather than later so that you test when you feel fully prepared and healthy. If you wait until you really NEED the test to be passed, there's a risk that something could go wrong - illness, injury, really bad nerves, etc.
However, there are plenty of valid reasons to wait. For myself, if I decide to do USFS testing, I would want to be prepared for both pre-bronze and bronze before starting the testing, so that I could save on the USFS membership fees.