I've been wondering this, but always forgot to ask when I was on the forums. Thank you for saving me the trouble.
Though I wouldn't throw out the idea of giving priorities to your own worlds completely- it can help to remind you what you need to defend. I'll often give priority 9 to homeworlds (mine and the AI), and then give 8s to any worlds with Advanced Factories or Advanced Starship Constructors. Then if I'm playing with the Spire Civilian Leaders, I'll usually give those worlds a 7.