No.I usually target mainly Google and I think its enough.Because most of the people use google and if we manage to rank well in google,then we can manage in yahoo and bing.All search engines are almost similar except some changes.I agree all the three have different algorithms and all but we can ignore once we manage to rank in google.If you have time then you can think or focus on yahoo and bing. And one more thing if your website is ranked well in google then you just need little efforts to rank in yahoo and bing.
