Always leave the valve open except when you're sailing is an excellent general rule. One caveat is: If your board is new you can leave the valve closed until you get your first ding.

Also, and very important, it is not the entry of water that wrecks a board beyond repair. All that means is that the board weighs a little more until you dry it out.

What wrecks a board beyond effective repair is the central core delamination (splitting of the core styro) that occurs when the styro plus moisture heats up while the valve is closed causing the pressure that separates it. That is what ruins the board. It causes the board to feel 'dead' when you sail it - a wind gust that used to translate directly to the fin and spur you on faster will no longer do that nearly as much. The reason is internal movement in the board. The deck and the bottom are no longer unified by the core and are relatively independent from each other. So, to avoid this fate, once you suspect your board may have moisture in your board, leave the valve open except when sailing.

A side issue is 'spinners'. Spinners are devices that spin the boards so that water is driven out holes drilled in each end of the board. One must realise that although they are effective for removing the majority of water from a board, they cannot remove it all. Water that is located 12" from the center of rotation (of the spinner) will have very little centrifugal force acting on it. So just because someone spun your board dry does not mean you no longer need to use your vent.