Bing came in at second place in February with a whopping 4.37% of the global search market. In case you didn't get that, Bing has a whole 4.37% of the market at second place. Who's first... I'm not sure, maybe you should "Google it".
Google comes in at 89.94% of the global market! While Bing and Yahoo may be creeping up they still have a long way to go before they can really be called competition. While there are a lot of SEO companies fighting for your dollars. It's important to have a clear idea of where most of your traffic will be coming from. Investing money in listings on small search engines may be valuable, but start with the big dogs first. Why invest hundreds in getting to the top of YP.com if 90% of your traffic is coming from Google.
Google Analytics will tell you where your traffic is coming from, so you can see if you are getting a bang for buck with the smaller referral websites. Keep in mind any ranking with paid advertising that is considered 'pay per click' or not natural optimization, will be lost after you stop making monthly payments. Natural optimization, may cost more upfront but it will keep you on the top long after you have stopped making payments.
So take a look at your rankings on Yahoo and Bing, and consider smaller directories but I wouldn't go investing too much time and money into optimizing for any search engine other than Google.