X feed valves are either Open or Closed, not On or Off.
The valve may be closed, but the cockpit control on my aeroplane says ON or OFF.
Bottom line - if the QRH says to close the X feed for landing, just DO it.
I think the point of the original post is if the FCOM doesn't say anything. Pretty obviously, if it says " turn it off for landing" then you don't have an option. But in those types where the FCOM doesn't say anything? It would be OK to land with it ON, and in fact depending on the fuel balance at the start of the approach, may be a good idea.