Q: my board has a vent plug. Should I open it when I'm not sailing?
A: YES!!
The difference between external air pressure and internal board pressure determines the exact thickness of the board. As the external pressure fluctuates every day (typically cool early mornings with high pressure, then thermal low mid-afternoons), the board will first compress, then expand as the day wears on. In a thick, high-volume board, this change can amount to 1/4" or more. Picture such cycling going on once a day, 365 days a year. Picture the EPS foam core fatiguing and slowly separating into chunks.
Much better to open the vent between sessions, to eliminate this cycling!