You take an Iron to a dry board?
Exactly you need to warm the board up to open up the pours in the base. This allows the wax to penetrate deeper into the base.
Also there is wax left in the center of the board from the previous waxing that you can spread around and fill in the edges again. Then just add a little wax for any dry spots left. I wax every time I ride a board. I have got 20 days or more out of a small bar of wax 65grams.
Not that wax is expensive or anything but why waste it by scraping every time I ride the edges get scraped dry by the snow. Not like I am racing or anything just out riding with friends.