I'm of a different school of thought: You should not work one muscle group every day. Even if they are not sore, or if you think you are not working them very hard, you are still working those muscles and tearing them down. If you don't give them a chance to repair themselves between workouts, you run an extremely high risk of injuring them with each subsequent workout. And the injury may not even be noticeable until it's too late...I mean, until you've really screwed things up.
Each time you work a group of muscles, the muscle fibers tear down a bit more (in other words, they tear...little microscopic tears all over and through your muscles). Maybe they don't tear enough to create soreness with every workout, but enough to weaken the muscle (because, let's face it, if you aren't tearing the muscle down, it will never get bigger and stronger as it rebuilds itself - it has to break down to build back up)....until eventually....RIP, you get a nice torn muscle injury.
Trust me, after that happens, you won't be working your legs for a could 2-3 months, and even then, when you start working them again, it will be another 2-3 months before you feel comfortable enough working them to capacity. I tore a calf muscle one year ago this week, and do you realize that even today, when I work my calves really hard, I feel that calf muscle ache something awful. I am realizing that I may never be able to work that calf muscle very hard without feeling that deep muscle ache that reminds me so much of the pain I had when I tore it.
The other nasty side effect of overworking your legs is that it puts undue stress on the bones and joints. As the muscles become weaker through the constant tearing-down you are doing to them, something else will have to compensate for that lost strength and provide support as you do those squats and lunges and other leg exercises. The first thing to go will be your knees. I have stressed my knees by overworking my legs. In fact, I just did it last week -- for the first time in about a year. I pushed my legs too far, and now my knees ache. It's a hard lesson to learn, and I'm hoping I can still engage in mild exercise instead of stopping exercising all together for a week or two to let my knees heal. By the way, if you are a younger person, you might not notice any of this "damage" you're doing until you get older. When you get older, all these things will start to manifest in your muscles and joints and could make your life very uncomfortable at times.
IMO, it's not worth the risk of injury to work your legs every day (or any one body part every day, for that matter). The only body part I've ever even heard you could consider working every day was your abdominals, but I still give them at least one day between workouts. You just aren't getting the full benefit when you don't give your muscles a rest period. They NEED to rest! And if you don't give it to them, Mother Nature will make sure you give it to them - unfortunately by way of an injury.
