It is very, very hard to the DM -and- an intelligent Wizard at the same time without seriously destroying your player's gaming experience.
On one hand, I can prepare my spells normally, which gives me a great advantage over everyone else, but my players would call me out and say that I'm using outside knowledge of the encounters to come to pick the perfect spells for each of them. (Seriously, though, when is having a Dimensional Door or Gaseous Form on hand at all times something a Wizard wouldn't just do?)
On the other, if I chose poorly to give them the illusion that I wasn't preparing for encounters that I knew were coming up, I'd be a grossly ineffective caster - just the same as the ones he's been in a party with prior.
With the first option, I'm preparing general spells that will come in handy during any situation. With the second, I'm intentionally choosing bad spells so that my players aren't dissatisfied with my character stealing the spotlight every encounter. It's a lose-lose situation.