The roads of the actual col itself will be closed early morning (0700-0900). The road leading there may be open later, I seem to remember driving along the race route on the way to the Col d'Aubisque in 2007. Not certain, though.
Remember it's better to be too early than too late.
I recall, from many years of watching Le Tour mountain stages on telly, that often the best spots for viewing get nabbed up to a week in advance.
I've watched stages in Paris & Brittany and along the route itself, the roads were closed to non-Tour traffic on the morning of the stage, and people were nabbing their places along the route from very early morning onwards.
Expect the road to be closed to traffic by noon. The caravane will start on Col de la Madeleine by 2pm and the riders will be there on top by 4.30/5pm. If you arrive late morning the only difficulty would be for you to find a parking spot because the higher you will go, the more difficult it will get to find somewhere to park (The road north face is narrow compared to south face where they will head down.) You should possibly try to park between St Francois and the top of the pass, and walk a little bit, this is what i'll be doing (I live in St Jean de Maurienne). Have fun !! Ced