In my experience, it doesn't make that much difference. My wife disagrees with me. The brown on guacamole is only from oxidation of the oil. It won't hurt you. You can either stir it in or skim it off, depending on your preference.
Some people say lemon prevents browning.
It takes a lot of lemon, but too much lemon changes the taste, unacceptably for me. The bad kind generally comes from animal sources. The avocado is America's answer to the Mediterranean's olive. Eating more monounsaturated fat, studies show, often benefits people with heart disease, diabetes and high blood pressure. One 8-ounce avocado has 400 calories.