Generally, I would say going into piedmont and making it sure marseilles won't be empty by autumn is enough for italy, to protect itself from a new fleet built in Marseilles.
Of course it's not always possible, to make Marseilles occupied by a French army in autumn, but still, it's better than doing nothing.
In Barn3tt game recently, Italy couldn't achieve this
gameID=83551In one of my games earlier, it was not just possible, but I actually prevented France getting a build.
gameID=82581As Italy, I ALWAYS open to Piedmont, and try to disturb France, later England/Germany do the rest of the work, resulting I can focus on Turkey. (never on Austria)