While proper ventilation is critical to a happy cruising life, it is important to remember that not only air can pass through your hatches and portholes.
The foredeck hatch is the most important hatch to maintain. This hatch is the furthest from the cockpit, so its situation often goes ignored. This hatch is also the closest to the bow, so it is the most likely to see ocean spray coming up and possibly into the forward cabin!
As a rule, we always close the forward hatch when raising or lowering headsails. I simply loosen the struts that keep it open and let the lid fall closed. I do this not to keep water out, but to keep the sails and all their control lines from fouling the open hatch and torquing it. Sheets and downhauls have a tendency to loop around the open hatch and present a problem while raising the sail. By simply closing the hatch, all of these problems are minimized.
Sadly, if you are going out in large seas, simply lowering the hatch will not suffice. A large wave over the bow will push a sheet of water over the deck. When this water reaches the unlocked hatch, it will force it open just enough to let water pour in through the hole. In our case, this pours right in on our V-berth, soaking the mattress and our pillows.
Now, our rule is to dog the hatch down as well as close it when raising sails. If the weather seems settled and calm, the hatch can always be opened. But if the weather is rough, we don't have to "remember to dog the forward hatch" as we are holding on to the teeter totter motion of the boat in large seas.
