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If you create the font-weight in a CSS file, and you don't give that CSS to aural browsers, then you're probably OK. Otherwise the user of an aural browser will hear every word on the page shouted at them.
If you make all the text bold with on page tags, you are also sacrificng one small tool in the SEO tool kit. The occasional bold word will get weighted as slightly more important than the rest of the text. So sometimes, even though my CSS file makes an entire passage bold, I will still place <b> or <strong> HTML tags in the HTML document, around important keywords in the passage.