I guess I would have to say that it depends on what site you're playing on. I know for a fact that on certain sites registration ends when the money I reached. For example, I would play in a particular tournament in the wee hours, that would frequently start with only five or six players in the field. Soon enough there would be enough players for a second table to be created and so on as the field grew.
At one point, for a week straight it seemed, this same tournament regularly started with only three or four players registered. If there were four players registered and one got knocked out, then registration was closed. If the tournament started with three players, registration would remain open. But if a fourth player registered and then a player got knocked out, registration would be closed once it field got back down to three players (the money). I specifically remember this because after it happened the first time and we saw what happened, we made it a point to try not to knock each other out until there were sufficient players in the field to insure that tourney would last long enough for more registrants to enter and thus build a decent prize pool.
Another thing that I specifically remember was that there always seemed to be certain players that would try to take advantage of the fact that players were making an effort not to eliminate each other, by frequently going all-in, knowing that they were unlikely to be called but someone who not only didn't want to get knocked out, but didn't want to knock the other player out either. In fact this is the only time that I can remember actively participating in stalling tactics.
Remember, if you play long enough, you will eventually see everything.