In the simplest terms: block level elements add line spaces before and after the tag inline elements don't
DrDoc
7:28 pm on Apr 12, 2004 (gmt 0)
<strong> makes no difference as far as search engine ranking. However, it provides the proper markup for a word you want to assign extra emphasis to. ...especially if a visitor is using a screen reader.
There is no need to replace <strong> or <em> with CSS if they describe what you want to do anyway. However, if the bold text is only meant as a visual effect then, yes, you should use CSS.