i dont want to book the ships excursions and have been researching booking shore excursions with private companies, but i was wondering if i'd be better off simply selecting a driver when i get to the port as we do in other locales?
I wouldn't. Europe isn't the same as the Caribbean. If you want to DIY those ports, then Rick's Med Cruise guidebook can give you pointers. But if you want a driver/guide for the day, then you would want to book this ahead if time. If you are on the Cruise Critic site, there are Roll Calls for each ship's sailings. Members on these often organize their own small group private excursions that tend to be smaller and less expensive than the ship excursions or going it alone with your own guide. It can be a good compromise.
You already got some excellent advice on your other thread.
I wouldn't use the same rules in all four of your ports.
Naples in particular has excellent public transportation for the most in-demand sites, i.e. Pompeii, Herculaneum, and Sorrento. Of course Do It Yourself creates a need to be back before the ship sails. You need to be careful that you don't book a tour that sounds comprehensive to you, but actually provides very little time in each place because it "appears" comprehensive. Month of year is a factor in this (i.e. local congestion), but you didn't give the month.
I would expect La Spezia to have extreme congestion, so that having the ship's excursion would remove a lot of stress about getting back on time.