While most web developers have been lead to believe that using the bold HTML tag adds value in the eyes of search engines, the truth is, they may as well be using regular font styles. The best HTML tag to use when your goal is to emphasize text is the strong and emphasize tags. These tags tell search engines not only that the font style is different but that the content within the tags is important.
While Google SEO has many components, using the strong and em tags help focus the topic of each page. You would always use the strong or em tags to emphasize the topic used in the page title and meta description tag to help your SEO ranking efforts. Careful strategy is important in assessing the frequency of use for the strong and em html tags. It is widely though that overuse will undoubtedly cause the opposite effect and actually hurt your SEO efforts.
My experience is as follows: Use the strong tag once or twice depending on the length of the copy content in each page. Typically I follow the rule of using a strong tag or em tag once per every 500 words. So if my pages are very long, I can use it a couple of times.
I tie the content of each tag to match some title keyword I'm optimizing for as well as the description tag content. Don't underestimate the power of the your header elements. I use the following rule when optimizing a page for Google SEO:
Use 1 - H1 tag per page, which matches keywords in the title and meta description.
Use as many as 5 - H2 tags which support variations of the H1 tag.
This guide should give you a good starting point to help with your Google SEO efforts. Keep an eye of the next guide, which will help expand on the topic of Search Engine Optimization.