Also consider the <strong> tag. It’s much better for screen readers and therefore better for accessibility. Search engines also use <strong> tags to determine important content similar to how they use header tags <h1>, <h2>, etc (although <b> will also have similar meaning to search engines). If you want to stress importance of text, use <strong>. If you don’t want to stress importance, use the <b> tag or use the font-weight:bold; style on the element or in the CSS.
Although, if you are bolding the entire paragraph, it’s probably better to use the CSS option. This will reduce the affect on screen readers and it probably doesn’t make sense to have an entire paragraph emphasized. But on the other hand, I’ve seen bold used to emphasize an entire paragraph before for good reason. In which case, font-weight:bold; is what you want to use, likely in a class/style.
In the end, <strong>, <b> or font-weight:bold; will all work and accomplish something similar visually (probably exactly the same), but they have slightly different meanings. Also, make sure that if what you’re bolding is a header, use the header tags: <h1>, <h2>, etc.