Dear I want to clear you search engine prefer user friendly content so if you are using bold italic and under line on important words then it will be easier to the user to understand the meaning of your post so Google gives preference to such type of content.
Bolding the selected keyword, helps the search engine spiders to easily determine the targeted keywords from other contents when they crawl the web page. So it is good to bold or strong tags on your targeted keywords.
Bolding a word has a very minute effect. I remember reading some results some time back from someone who did 100,000 tests to determine how html tags affected search engines and bold word fell into the lower end of effects.
Bolding a keywords is nearly useless, but still have some SEO value. As with headers, you better use it for the benefit of your human visitors. But do not put every 5th keyword in a bold text as it looks ugly while not giving any significant boost to your rankings anyway.