Nope, the rules do not prevent this:
Aiming a Spell:
For an Effect: Some spells create or summon things rather than affecting things that are already present.
You must designate the location where these things are to appear, either by seeing it or defining it. Range determines how far away an effect can appear, but if the effect is mobile, after it appears it can move regardless of the spell's range.
Burst, Emanation, or Spread: Most spells that affect an area function as a burst, an emanation, or a spread. In each case, you select the spell's point of origin and measure its effect from that point.
A burst spell affects whatever it catches in its area, including creatures that you can't see. It can't affect creatures with total cover from its point of origin (in other words, its effects don't extend around corners). The default shape for a burst effect is a sphere, but some burst spells are specifically described as cone-shaped. A burst's area defines how far from the point of origin the spell's effect extends.
A spread spell extends out like a burst but can turn corners. You select the point of origin, and the spell spreads out a given distance in all directions. Figure the area the spell effect fills by taking into account any turns the spell effect takes.
A Fireball is a 20 FT Radius Area Spread Effect Spell with a minimum 400 FT range.
It doesn't state anywhere that you can't curve the delivery of the spell in anyway so long as it's in range. Thus curving the spell is allowed. If you want to houserule that in your own game go for it but RAW it is allowed. Think of it this way, if a MLB pitcher can throw a wicked curve ball without the use of any magic then why can't a wizard shoot an epic curve fireball with magic?