You'll waste money and pollute your local water by using herbicides against them. Are they really that ugly? I suggest giving it up, and learn to love the pretty flowers. Their deep tap roots will keep your yard from becoming a big flat impervious sheet. They have culinary and medicinal uses, you can read more here:
http://en.wikipedia.org/wiki/Dandelion
or use your favorite search engine to find dandelion uses.
Good luck, and take it easy with those herbicides...