I only close mine if a storm is expected (not that this should really matter).
I figure that for valves/hoses/ clamps that have been regularly checked that the odds are very low of a leak "just happening".
The closest my boat has come to sinking due to a thur valve line is the thur hull
fitting itself completely corroding away to the point that it was being held in by caulking (it still wasn't leaking, yet). So I guess if you close them regularly you would notice the fitting loose in this case (didn't on the 1 I had the issue with as that one was opened/closed weekly and I still didn't find the problem till haul out).