(Disclaimer: I've never been to the WSOP, this is just based on what I read and how other tournaments work)
It depends on the event. Most of them keep registration open until the end of the second level, so it's only really a problem if the event sells out. If you wanna play a $2K PLO8 event or something you shouldn't have any problems. The $50K HORSE only had 30-something entrants at its designated start time, but they ended up with around 90 who registered at the last minute.
For one of the $1500 NLHE events that draw massive fields though, you might wanna consider preregistration to make sure you get a seat.
Queues might also be an issue, as mentioned above.