The accuracy of the Find My service on any Apple device depends on whether your iPhone is still powered on and is still connected to the Internet. However, if your iPhone is already dead, then the last known location of your iPhone is the one that it sent when it has low battery. But even if your iPhone is still shown on the map, it will still take time finding its exact location. However, if it's just inside the house, then you really don't have to worry about it too much because you know that someone will still be able to find it as long as the device never leaves your home. For an iPhone that's still powered on, you can also just dial the number associated with it so you can easily find it when it rings.