In general, there are two issues with this:

1. You can't have checked bags. However, that doesn't apply when you enter the U.S. because you have to pick them up and clear customs anyhow. You just "forget" to put them on the re-check belt as you walk out.

2. In case anything goes wrong with your original flight, the airline is within its rights to rebook you on any other flights that get you to SMF. Since there aren't any direct FCO-SMF flights, you'd have to connect somewhere. Odds are this would be SFO. In the unlikely event that this happens, and they suggest a routing to SMF via LAX or somewhere else, you're allowed to discuss things with them and see if they have something better (for you). To deal with this eventuality, you might want to have a list of alternate flights in your pocket. In your situation, worst case is they send you to SMF via East Overshoe and you're still OK. In other situations, not connecting through the right intermediate city can be a bigger concern.