That's entirely up to you, and how you want to run your contest.
Most people (99.999%) post the numbers after all of the boxes have been sold. This provides the most fairness, and puts all players on the same playing field in terms of luck.
Having said that, you could post the numbers right in the beginning, giving a sort of "first come, first served" approach to square purchases. Although this will eliminate the fairness, and many boxes with poor numbers will likely go unsold.
An added benefit to posting the numbers early is that it helps you sell squares in a second contest, as players who were unlucky and received traditionally bad numbers will be more eager to join a second contest and try for better numbers the second time around. This obviously will help you raise more funds for your cause, or more fun for your friendly contest.
We strongly encourage you to post the numbers as soon as you possibly can, as players are always eager to see what numbers they got and the longer you wait, the less transparent the process seems to some players.