Christine,
When you are dealing with Hertz, Avis, or other names you know... you must realize that many of these are Italian owned. And many of the smaller vendors are franchisees. Thus, there are multiple layers when dealing with issues. You don't call Hertz USA... you have to call Italy and talk with Hertz Italia!
We did not own a car when living in Italy so we rented often. Like Roberto, we ALWAYS took the $0 deductible option. Even then, NOT everything is covered, like UNDERNEATH the car, tires, windshield, etc.! I had a dispute that went on for 6 months with Thrifty in Rome until they admitted they had the wrong file and wrong car! I had already cancelled my credit card, etc. to protect myself. They offered no apology or compensation. What a pain!
But as mentioned, do walk the car, take pictures, and MAKE SURE the staff on hand acknowledges what you find. Have them initial everything - and don't be reluctant to be demanding ... because this is where you protect yourself. I had one car that was so badly scratched I had mark-after-mark on the page. They finally just gave me a new car, they were so frustrated by my need to acknowledge the issues. I then took the same time with that car! Protect yourself -especially if you have a one-way rental.
Hertz cannot add the $0 deductible insurance here in the USA; it must be done in Italy. Avis can, but you have to prompt the reservation clerk here in the US; Auto Europe does perhaps the best job with $( deductible insurance, but you will have to prepay. You can cancel up to 3 days in advance when purchasing AE online - or if you work through a travel agent, you can often cancel up to the date of your rental. Your money is then returned to your credit card.
Make sure you get the IDP for each driver, and do plan on how you'll coordinate your GPS needs. A GPS options may be more limited on one-way rentals. I book a lot of rentals now for folks and AE often has the best prices, you can pay in US dollars, and I've had good support with issues. Hertz has probably been the group I've booked as an alternative. I would stick with an international name you know, and one that meets your drop-off and pick-up options. Venice is a GREAT place to pick up a car, because it's such a destination location. Many folks get free upgrades here because the vendors want to get cars back to Rome, Florence, Milan, etc.
Good Luck!