when should i sign out of my apple id?

Only if you are selling the device. If you are selling the device, giving it away, or otherwise transferring possession, follow the steps in this article: What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support

I never sign out of my Apple ID. I would do so if the computer/device was going to be repaired or if I was giving it away/selling it.