Water that has sat overnight has no more free chlorine, even if you do not add prime. If your water department adds chlorimine, it may have ammonia left in it, but a well established tank should be able to absorb the ammonia that would come in a 10% water change. If you add prime it will detoxify that ammonia for an extra level of protection.
0.1 ml of Prime dechlorinates and detoxifies 1 gallon of water, in just a few minutes. I use a .5 ml oral syringe to measure mine, I'm guessing 0.1 ml is 5 drops or so?
If you want to be scientific about it, you can measure the ammonia level in the bucket to decide if you're adding enough prime. The side of the prime bottle says how much ammonia is detoxified by the standard dose. (Your kit will still detect it, but it will not harm your fish.)