<center> is to be avoided, because it mixes up style and content, which must be kept separate at all costs.
But <p class="centered"> plus .centered { text-align: center; } --- now that's some clean code!
Likewise, you should always use <em> and <strong> instead of <i> and <b>. Someone might want to emphasize (or... strengthen?) parts of your content differently than with italics or boldface. In this scenario, they can't just style the <i> and the <b> exactly the same way one would style the "semantic" versions, for... reasons.
A good rule of thumb is that the more you have to type, the more semantic your HTML is. Hence why we can no longer use <blink> or <marquee> and instead must write god knows how many lines of CSS to accomplish the exact same result. Typing is a ritual we perform to sanctify our HTML. The more you type, the more blessed your website.