I didn't actually go with color limits, but I thought the limited palette would give it a certain flair. The way I understand it(I'm totally not sure), each sprite is allowed three colors- and some may appear to have more because they are actually more than one sprite stacked on top of eachother, on different layers. I think that's the case with the NES Megaman sprite.
Obviously it's a matter of taste, but I don't personally feel that strict palette limitations are necessarily useful. I understand that some retro-game artists could probably spot when you are using an extra colour, but elevating colour usage to the level of a religious observance is going to be lost on most people.
That said, colour consistency is still a must in video game sprites, and that's not necessarily a bad way of policing yourself. But, basically, I wouldn't lose sleep over a little violation here or there so long as it's in good taste.