WotLK roll for my off spec (ninja or not?)

"Ninja" is defined as deliberately breaking pre-agreed looting rules, to gain something that you did not deserve to win, and the pre-agreed rules defined you should not have taken.

In your case, this depends on what the rules were agreed on beforehand; Generally if you're talking about 5 man dungeons, no one gives a damn if you roll need for offspec, and in fact, this is often done especially when there's a disenchanter in the group as the disenchant roll takes the same priority at greed.

It is however considered common courtesy, that IF you rolled need for offspec, and won an item, that you offer it to anyone else who needed it for their mainspec. This isn't reinforced however, and not everyone will do this. For example if you're a tank and you roll need for a DPS item, it's considered polite to give it to the other DPS who's using it for mainspec - Although if you decided to keep it in this case, it's not considered ninjaing, just rude (also be prepared for them to hurl abuse and ignore you and whine nonstop)

A good example of ninjaing is if something valuable drops, a common example being trinkets in Icecrown Citadel, where the master looter does "Roll for Deathbringer's Will", rolls the item, and instead of giving it to the winner, loots it himself then logs off with it. In a case like this, then yes, this is very muchly bannable.

Another case of ninjaing is if someone asks for a profession crafter, or gem cut, or something similar. For example, if you ask a Blacksmith to make you some Pillars of Might, which is a tanking 264 BoE; You trade him the mats, the blacksmith crafts the item, and then the blacksmith logs off with it and doesn't trade it back to you, then again, yes this is a ninja.