South of France to Northern Italy

You need an IDP from your local AAA office.

Renting a car in Europe, if not returning it to the same country there is a huge fee, as the car needs to be returned, probably on a truck. Read this about driving in Italy and restrictions involved [ZTLs]; avoid leaving anything of value in a vehicle when parked:

http://driventoit.blogspot.com/

I would be picking up in France and dropping off in France. Just not sure why some people say IDL is needed and some not :)

Because some people like to live dangerously--or suggest that others do so. There's the potential of a very large fine if you're stopped in Italy and don't have the mandatory IDP. Some folks think because their car-rental agency didn't ask to see an IDP, it's not required; that's not the case.

Just not sure why some people say IDL is needed and some not :)

No idea why some people say it not needed. The only acceptable option to an IDP is an "official" translation of you license into the local language

It is really quite clear it is needed ( the wording for France is in essence, the same)

https://it.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/transportation-driving/

Seems my CA license should be enough, am I correct?

No

"Just not sure why some people say IDL is needed and some not :)"
Because some know what they are talking about and others....not so much.
Get your IDL. Sometimes you need it (someone wants to see it) sometimes not--just how lucky do you feel?

Even if you just stay in France it is a requirement unless you have gone to an approved (read - few - expensive) translators and had a translation into French made and stamped. Much easier to spend $20 and a photo at AAA which are all over.

And yes Italy requires it too. And Italy makes random traffic stops.

Get it.

Some people say don't because they hate authority. Doesn't make it less required.

Maybe some people are cheap or lazy? It's a drop in the bucket compared to flights, hotel, rental expenses.

It is also very easy to tell other people not to get it. They aren't the ones paying the penalty.

... and another thing, you will find rental cars are crazy expensive now, so you should add a $1,000. dollars to your budget.

When you say "south of France", where will you actually be? It will have an influence on the best route to Italy!

I will be leaving from Grasse to Italy. The rental cat through AutoEurope was $500 for a month. Thanks all

Good luck as you travel. We were in Nice, picked up a rental car and traveled a relatively short distance to the Piemonte where we stayed in Alba. It was the perfect base for visiting the surrounding wine villages.
We drove to Lake Garda next, spent a wonderful night or two in Salò on the way to Ortisei in the Dolomites. When we left the stunning Dolomites, we stayed on the other side of Lake Garda in Malcesine. Then continued on to Bologna. We returned our rental car to Nice( much less expensive than in Italy) stopping in charming Noli overnight.

Thanks Suki, great tips. How long was your trip and what time of year?

Suki's suggested route is excellent.
Beware that the shortest route between Nice and Piemonte (Tende tunnel) is closed until further notice due to massive storm damage. Thankfully, the alternative route via Savona is not much longer and quite scenic too, despite being all on major highways.

An IDP is required to legally drive in Italy. Should you get in an accident without one, your insurance could be nullified.