The problem with opening to Piedmont is essentially twofold:
1) A move to Piedmont is a severe nuisance to France, but only a nuisance; you don't get centers out of it. One unit around Marseilles is all that's ever needed to deal with it, and after 1901 that will be easy for any France that isn't in the middle of dying at the hands of the E/G.
2) On the other hand, if France sends a lone fleet to annoy you, France won't get centers out of it, but it's potentially deadly. After all, you only have 4 natural units, at most two fleets - beyond that, you have to fight a winning war. Turkey will tie up at least one fleet, and often two. If France walks into Tunis, then even if you kick him out, you lose the Ionian to Turkey, which is often fatal. If France walks into the Tyrrhenian, you lose something, followed by the Ionian, which is often fatal. And to cover both takes two fleets, and you can't spare two.
In other words, annoying France is likely to provoke retaliation, and France can do a lot more damage by pestering you than the reverse. Don't annoy France unless you're prepared to do a lot more than just annoy him. :)