I know something about search engine optimization. All I want to say in this post is that I have had my websites in Google, Yahoo, Msn, Ask, dogpile, etc. I could go on but the rest of my search engine traffic accounts for less than 5% of my traffic/income.
Google is the best search engine then Yahoo and Msn comes. so if your site has top ranking in google you will get more traffic.
I agree with the point that Google commands the largest share of search traffic and hence seems to be the likely choice. However before deciding on the search engines to optimize for, it would be a good idea to segment you traffic source by search engines. Base your decision on this information and it would be the right one for you.
If you're getting 95% of your traffic altogether from google/yahoo and MSN you should not bother about the rest 5% of traffic from other search engine.