I woud say don't spend your IP on champions. Runes at tier 3 are hella expensive, and it's really to your benefit to have a full page of tier 3 runes over x amount of champions you could have purchased.
Just as an example, I had around 16k when I hit level 20, and not even a full rune page set me back to 2k. Purchasing champions, that would have got me 2, 6300 champions and a 3150 champion.
However, I now have extra health for all champions I pick, along with armor penetration or magic pentration depending on which page of runes I pick. Paired with the appropriate masteries, it makes a noticable difference.
Then again, I spent over $100 on champions, so I never really needed to spend IP for champions.