Ciao Christine,
Yes, I would reassure you it will be safe to park and get back to your car after 8 or 9pm, and that is because dinner traditionally is late across all of Italy. Restaurants generally open around 7pm with most guests arriving around 8pm for dinner. So that means that everyone, visitors and residents alike, are out and about at those hours. Dinners out generally take about 2 hours on average. Actually, parking lots might be quite busy all things considered. With people around, you don't have to worry. October is still a busy month across Tuscany, while there are more rainy days, temperatures can still be good and lots of people choose to travel throughout the autumn months. Just bring layers so you can take them off easily during the day and cover back up in the evenings.
My more general recommendation for safety in parking lots would be to avoid leaving suitcases in the car at any time of day. Unfortunately, busy parking lots with lots of cars coming and going also attract attention from thieves who do break into cars to try to get any valuables people might leave in the car. So as long as you avoid doing that, the car should be okay while you're out for dinner and with people around until later in the evening, there is also less possibility of anyone having a chance to lurk around and wait for unsuspecting victims. I'll say that is most common in Pisa, because with the airport nearby, many head there with all their items and then think to make a "quick stop" to see the Leaning Tower before leaving (or on arrival before heading elsewhere) and then their vacation is ruined because their car was broken into. This is not very common across many other towns in Tuscany, but better safe than sorry.
All of the towns across Tuscany that we have visited have parking lots outside their "walls" or "historical center" where you can park and then walk in. This is because most of the historical centers are protected: by keeping cars off the small streets, not only do they protect historical buildings from smog and traffic, but all visitors can stroll along the streets without worrying about being run down.
This is in general for the smaller Tuscan towns: larger towns like Florence, Siena, Lucca and Pisa are also off limits to cars in general, but the people who live and work inside the centers are allowed to get permits to drive into town and so the streets are not actually car-free. Driving into these "protected" areas (called ZTL areas) are prohibited and photos are taken of cars driving in, so watch out that you don't go into any ZTL area because you will receive a fine months later back home (drivers are tracked down through car rental agencies).
In any case, as you arrive into the towns, follow the square blue signs with a large "P", sometimes the arrows and names of the lots are on white signs below the "P" to indicate where to head to. Both Montalcino and Montepulciano are lovely, both on top of a hill so there is a bit of walking uphill to go after parking but not far. I'd also recommend making stops in Pienza and San Quirico once you're there. Will you be staying in the area for those few days? We love staying in San Quirico because it is very central and then we generally head out in one direction one day, in the opposite on another day. So if you haven't decided on where yet, that's my top favorite area to base ourselves in to visit all those towns. If the weather is nice, you should consider heading to Bagno Vignoni too as a short stop, where you could try dipping at least your toes into the thermal/hot spring waters. Read more about Bagno Vignoni here:
https://www.discovertuscany.com/valdorcia/bagno-vignoni.html
Let me know if you have any other questions!
