Below are the correct usage of the four elements to choose from. I would say if you are marking out something as important, you should use strong. Markup your content correctly. You never know the benefits. The content could be consumed by a screen reader, a RSS reader, insta-paper or some other uninvented device in the future.
Your Sites stylesheets should then take care of bolding the marked up words.
Usually marking your content up in a semantic way as possible has a net positive affect on SEO too, but a few strong words probably won't proppel you to number one on their own.
i — alternate voice such as transliterated foreign words, technical terms, and typographically italicized text
b — “stylistically offset” text, such as keywords and typographically emboldened text
em Stress emphasis, i.e., something you’d pronounce differently
strong — strong importance