In general you will want to market your site to different search engines. There are strategies for doing this optimally. One question might be which search engines are the most important.
Search engines usage exhibits much of the market concentration or 'flight to quality' that you see in other internet related industry segments. The majority of searching occurs on three search engines. Therefore your efforts towards search engine optimization should be focussed on these specific firms. Efforts towards other sites will have only nominal results.
According to Search Engine Watch / Nielsen Ratings the breakdown is as follows:
This exhibits the classic 80/20 rule. Eighty percent ( actually 81.1 ) of your results will come from 20 percent ( 3 sites ) of your efforts.