"At the peak of concern, Apophis asteroid was placed at four out of 10 on the Torino scale - a measure of the threat posed by an NEO where 10 is a certain collision which could cause a global catastrophe. This was the highest of any asteroid in recorded history and it had a 1 in 37 chance of hitting the Earth. The threat of a collision in 2029 was eventually ruled out at the end of last year.
Alan Fitzsimmons, an astronomer from Queen's University Belfast, said: "When it does pass close to us on April 13 2029, the Earth will deflect it and change its orbit. There's a small possibility that if it passes through a particular point in space, the so-called keyhole, ... the Earth's gravity will change things so that when it comes back around again in 2036, it will collide with us." The chance of Apophis passing through the keyhole, a 600-metre patch of space, is 1 in 5,500 based on current information."
I suggest that when the asteroid passes by in 2029, we all get really, really drunk and hope for the best. If things go the wrong way, the whole planet can panic and solve the problem in 7 years.
I just saw a program on the National Geographic channel about asteroids. They said that you are 4 times more likely to die from an asteroid impact than you are from a lightening strike.
If you do it in the most sensitive direction before a near gravitational slingshot past a larger body, you change its course through the deeper gravity well. Effectively, the course correction to put in before, is amplified by the next interaction. The best time to get one is several years before a close pass, letting the ship sent (which can be robotic of course) hover over it for several years. Even a small mass can do the job with enough lead time.
So no, don't wait to see if it is too close after the pass, calculate well and deflect years before the 2029 pass.