Chris,... I have read that you want to keep all seacocks closed in the winter except the cockpit drains, but I'm not 100% sure about the underlying theories.
The closed thru-hulls admonition is for boats that winter in the water. The precaution being one of not relying on hoses and hose clamps to keep out the sea when a seacock or valve is the better alternative. Cockpit thru-hulls need to be left open to allow rainwater or melting snow & ice to egress weather on the hard or in the water. There are those who have opinions about thru-hulls open or closed while on the hardstand as well - but those are a matter of preference rather than safety.