Ciao Jespoh,
What I would do is start by writing out your full itinerary for ITALY - if you want to share what you have set down, I can offer more insight into whether the car rental makes sense or not. After that, figure out whether a car rental is necessary and where.
For example, if you're visiting the main cities for the most part, you could consider traveling on TRAIN between the cities and only rent for a portion of your trip. Train is generally a great way to travel (there are the occasional strikes, but timeliness is pretty good).
If you're going to be in Rome, Florence, Venice, Milan, Naples - I would definitely say you DON'T need a car while staying at each of these cities. All of the main attractions are centrally located and within the historical centers that are "limited traffic areas" as far as cars go (the dreaded ZTL areas require a pass for your car that mostly only residents and those who work in the area can obtain). You won't be able to enter those areas with the car nor park within the area unless you're willing to pay high fees at private parking garages within the center that clear your license plate from the lists of those cars that enter the ZTL area without a permit. So you would be paying daily for the car as well as for the parking to have the car - not in use - while you visit each city.
On the other hand, if you plan to spend more time in the countryside, such as exploring Tuscany or staying outside of the cities and doing day trips into the cities - then you would benefit from having a car. You could plan to use train between cities, and then, for example, rent out the car for a piece of your month long visit. If you're in Florence and then plan to also visit Tuscany, I'd say first stay in Florence, and only once you're ready to leave Florence rent a car as you leave and go explore Tuscany. At that point, you could keep the car rental even beyond Tuscany if you're headed elsewhere in the countryside - or return it if you're headed to another city center. It really depends on where you are headed, the order in which you've planned your trip.
Let me know and I can offer some more suggestions or advice!
