Is there any benefit to using outdoor faucet covers if the water to them is shut off?
Metal pipes conduct heat very well.
You have another question indicating that you have been insulating pipes.
If a pipe is insulated from the warmth in the house and has a turned off hose bibb sticking out into -10F weather, it's far more likely to freeze if that hose bibb is not insulated from the -10F weather.
In any case, given all your other posts about insulating the house, even if the pipes don't freeze here's a heat leak that's easy to fix that you'll be paying for even if it does not freeze the pipes.