I had no issues leveling my teams with different specs. Out in the world, things die so fast and leveling is so fast that it doesn't really matter what your setup is. Most of the time I don't even bother with upgrading spells or macros for 10-20 levels at a time.

Like Knytestorme says, there's going to be spec-specific rewards coming with the MoP changes so it's probably going to be a better idea to just level as the spec you want so you're getting upgrades that you'll use. On the other hand, you could always level all the same until you get to 80 or so and then choose a final spec so you're high level quest rewards are useful. It depends on how you want to level. IMO -

As Fast As Possible - spec them all the same and don't worry about it until you're nearing the end of leveling.
Slow enough to hit dungeons on the way - choose your preferred spec early so you get the best gear you can on the way.