1. Depends on what you're hoping to see. The start can be a bit of a circus, but if you get there early enough, it's a good chance to see the mechanics prepare the bikes, the riders exit their compounds, sign in, etc, but they usually don't start racing until a little ways down the road from the "official" start.
2. If that's what the schedule says, then that's when it will happen. For such a huge event, the Tour runs like a precise military operation. Most stages start in the early afternoon to ensure the finishes will be in Prime Time for French television, around the dinner hour.
3/4. If you're going to the start or finish of a stage, it's good to get there early to secure a good spot along the barriers. Along the route, you have a lot more options. Study maps of the area where you want to go so you can figure out a route in and out, as the road on which the riders race will probably close to public traffic a couple of hours ahead of the racers to allow for last minute safety inspections to make sure there are hay bales against sign posts, etc, and then, of course, the ever-popular publicity caravane.
5. Absolutely! Bring a picnic, some wine, and make a day of it. Hang out with the locals, talk cycling with people from all over the world. Ideally, look for someone who has a portable tv with them, so you can catch glimpses of the live tv coverage as the stage approaches!
6. Have fun!
