The Sheraton I stayed in Rome had a shuttle to the airport and the piza venize which is almost in the middle of most of the attractions (a few blocks from everywhere). I noticed at the airport there were several other hotels such as Hilton offered shuttle services as well. You may want to call Starwood to check out if there is shuttel services offered by the St Regis. It definitely helped us and saved us the time to figure out transportations when you have short time and do not know the city at all.
Also try to get hold of a good local map if you like to walk around and explore. The map we got from the hotel was much helpful than the one we got from a travel book.

Enjoy your trip, definitely try some gelatos at the shops by the Treva Fountain. They have great selections of flavors.