The start is a *great* place, just make sure it's a regular long ride day and not a time-trial!
Usually just behind the starting line banner is a huge stage with a table on it, which is where the riders sign in - ALL OF THEM. So if you can get yourself on the fence near the stage, you will see ev-ery-body. It's a great place to get stuff signed, too, if you're into that. Everyone is pretty calm and relaxed, and you'll see team cars, photographers on motorcycles, and hear a few interviews with guys as they go up to sign in.
When the signal to go is given, it's a neutral start, so you won't get to see any real racing. But you will get to see plenty o' spandex for sure!
-- gnat! (I recommend starts, KOM tops, and seeing at least one sprint finish if you can!)