The parade will shut down Constitution Ave for much of the day which will back up traffic on the TR bridge. If you come from IAD/DCA, you may want to tell the taxi driver to take the Key Bridge instead. Otherwise, Metro is probably your best bet.
The fireworks (starts at 9:10pm this year) are launched from the base of the Washington Monument. If you can see the Monument from your hotel, you can probably see the fireworks too. Most people sit on both sides of the reflecting pool between the Lincoln Memorial and the WWII Memorial which has a dead-on view of the fireworks. The St Regis' advantage is its proximity to the Mall (the closest Starwood to the Mall, perhaps?) - about 10 blocks or so. Walking to and from the Mall for the fireworks display is advisable as Metro will be uncomfortably packed, if you can get on a train at all.
Have a great time and hopefully the weather won't be scorching this year!
