Some keywords should be bolded to point the search engines in the right direction.

When you are writing your web pages you will probably want to give importance to a certain word or two. Unlike header tags, using the <b> option in your html code to make a word stand out doesn't carry as much weight with the search engines. Don't discount it though because it does matter.

Any time you stress a word and make it stand out you are also stressing that word to the search engines.

Bolding the Text should be applied whenever it fits into the flow of your content and will match the look and feel of your website.

Many WYSIWYG (What you see is what you get) programs do this for you on the fly, but make sure you end up with clean website code.