Once they all start indexing your pages you will notice Yahoo actually crawls your website more often than any other search engine out there. For this month alone Yahoo has crawled my site in my signature 3333+102 hits and Google comes in at 1975+32 hits. All I did was submit my website to Yahoo. Give it time they will start indexing your webpages. My opinion is to concentrate on getting content on your website and worry about what gets indexed later.