Use bold for call-outs, subheads, etc. Don't bold whole sentences or random words in the middle of a sentence. If everything deserves emphasis then nothing is truly important and the reader has no visual cues.
Example taken and modified from "Scratch THIS, Pussycat!"
Bad:
Cat lovers often face a quandry when they also love their nice furniture and thick, lush carpeting. No one wants to live with sofa arms and drapes shredded and tattered, or carpet strings pulled up, yet few of us are willing to give up our furry family members either.
However, you must put yourself in kitty's boots for a few minutes. Cats do not scratch furniture with malicious intent. It is part of their regular self-maintenance program to keep their claws nice and sharp for self defense. Actually, they are not "sharpening" their claws, as such, but dislodging and removing a transparent sheath that grows over the claws. You may occasionally find these sheaths buried in your carpet. Yelling at your cat or getting mad at him only confuses him, because he is doing what comes naturally, with the nearest tool at hand, which may presently be your prized Louis IVX chair you inherited from Aunt Blanche.
The bold in the example above provides emphasis, but offers readers no visual cues to help them scan. The reader feels like she is being screamed at, rather than guided through the text.
