I am always keen on watching the meter and paying the meter on arrival to my destination, but I have noticed on several occasions where the driver touches a button on the meter after the vehicle is stopped and the fare jumps slightly. I have noticed this particularly in Rome, but also in Florence. I always round my fare up to the nearest Euro and tip 1 Euro per bag, but have never gotten a satisfactory answer when I inquire about this jump in the meter after the taxi is stopped.
It may be the "bag charge." Generally (on the taxis we have ridden in spain at least, and I believe also italy) there is a window sticker explaining the charges. A bag charge would be added at the end, not at the meter drop.
Probably related to fees for extra passengers or luggage. Sometimes that is added at the beginning. Don't be shy, ask.
It probably is an extra baggage charge afterall. It seems that this occurs more noticably when we are on the move with our "stuff".
the button adds in the extras - so don't feel like you have to tip beyond rounding up to the next euro. the real problem is making sure the meter starts correctly! catch a cab at a stand, not at the corner. If they've been driving you'll end up paying for the non-existent passenger. At the taxi stand you can see the meter before racing into traffic.
I believe in Florence you can see the extra charges even before the driver pushes the button. There is the meter based on distance on the left and the surcharge amount shown on the right. The button push adds the two together. There are charges in Florence for each bag the driver puts into the trunk for example and this charge will show on the right side of the meter as a fixed amount that doesn't change during the trip. There might be other legitimate (as in posted) surcharges in Florence, but I can't remember them right now. In Rome I know there are extra charges if you call a taxi to pick you up for example. Again, this is legitimate (as in posted).