If I was dominating a particular niche and felt the opportunity was their to take spots two, three and four as well, I'd jump on it.
You'll only be taking so much of the traffic by ranking first, why not have the other main traffic too. Remember, eventually those that are in the positions below you are going to come gunning for your spot at one point or another. This also doesn't take into account a potential slap from Google, leaving you with nothing if you're only holding one position.