If you use only NTLMv2 and dont store LM hashes in the database, the only way precomputed hashes (Rainbow tables) would help would be if an attacker already has admin on an AD server and can dump the NT hashes. Also, the NT hashes unlike LM hashes do not break into two seperate 7 char hashes, so the space requirements for NT hashes(MD4) is going to be a good chunk for 14 characters. I would say strong password policy is a much better investment than worrying about cracking. The strength of the hash is irrelevant when it comes to guessing a weak password.

-Maestr0