In most of the ground beef recipes for the slow cooker, you'll want to brown the meat ahead of time, so that your food is not cooking in grease the whole day.  Of course, things like meat loaf and patties that can't be browned ahead of time, as easily, it's OK to do in the slow cooker, just select a leaner style beef, or put a rack or foil balls underneath so that the meat isn't frying in it's own grease all day.