We get asked occasionally about optimizing for Yahoo, Bing, and others, rather than Google.
As with any expenditure of time and money, you have to decide where to do your spending.
As of today, Google owns the search world, without question. As you can see from the following graph, Google has 85%, Yahoo and Bing split 10%, Baidu (Chinese language search engine) has 3%, and the remaining 2% is divided among thousands of search engines competing for the crumbs... so why even think about anything other than Google? Go where the searchers go!