Hello, it's my first time on ChowHound...
I'm from Rome and I live/work in Southern Tuscany since 3 years. Looking at your post I undestand that you are already in Italy, and I hope it's not too late for my suggestions..
These are the restaurants favourite by (us) locals:
OSTERIA ACQUA CHETA ( av. price 25 Eur/p.p) for "bistecca fiorentina "and local food.Hand-made pasta ( great "pici", typical hand-made pasta of Montepulciano area) and bread. Reservation is required, especially in high season, or you do not have any chance..
LA GROTTA. Wonderful location ( just opposite San Biagio Cathedral).Maybe the best place in town and the surroundings. Opened since about 15 years, family-run,Italian cuisine based on local food, great service, wide wine list. The owner, David, can assist you in the selection of wine and has a deep knowledge of Italian and French wines.Average price 45 Eur/p.p
LOGGE DEL VIGNOLA.
Cosy restaurant ( less than 20 seats) in the historical center ( Piazza delle Erbe), good service, perfect for a romantic dinner. Great selection of Italian fine wines. Massimo , the owner, is a sommelier . Av. price 40 Eur p.p.
E LUCEVAN LE STELLE.
Bistrot with a panoramic view over the Val D'Orcia, in Piazza San Francesco,one of the most beautiful squares of town. Perfect for a light lunch ( selection of local salami, cheese, pastas..) , for an aperitif on the terrace or for a wine tasting. Wide selection of wine by the glass (usually they have 15-20 different wines by the glass) Live jazz in summer time. Av. price 15-20 Eur p.p
Valiano di Montepulciano
PICCOLA TRATTORIA GUASTINI
Located in Valiano (about 10 km outside the historical center). Not touristic at all , about 20 seats, local and fresh food ( also hand-made pasta9), young owners. In summer, a small terrace. Av. price 25 Eur p.p.
LA PORTA. Few metres after the gate of Monticchiello.Local food, very nice atmosphere , great place in summer time ( they have a terrace with a view on the valley). Maybe a little bit overpriced for the service ( av. price 40 Eur p.p.), but the food is great
TRATTORIA LATTE DI LUNA
Family-run and typical trattoria. The owners ( Roberto and its wife) are really the "soul" of this place. You can eat wonderful "pici" and an unforgettable "maialino al latte". On summer, you can eat outside, in one of the most beautiful corners of Pienza. Reservation is required ( always full!). Av. price 25-30 Eur p.p.
Near the rail station ( Via Gramsci).It's very difficult to find there any tourist, it's a place known mostly by locals.Forcillo is opened since 1950 ( or something like that), it's a family-run trattoria. Great "acqua-pazza soup", good meat. Avoid the "antipasto della casa". Limited selection of wine, very good rate quality/price ( av. price Eur 25 p.p.)
IL CONTE MATTO. Located inside the Castle of Trequanda. Perfect for a Fiorentina/Chianina steak.Very nice on summer /spring ( garden).Av. price 35-40 eur p.p.
LA BOTTE PIENA.Located in the main square of this small village, the restaurants offers a good selection of local cheese, salami, meat. It's not a formal place, but they have a good service and it's not expensive ( av. price 20-25 Eur)
In our area food is based mostly on meat ( wild-boar, pork, veal) and vegetables. I would never suggest to ask for fish. Anyway, if cannot resist without fish, one of the rare good places is RISTORANTE PATRY, in Chianciano Terme ( near Terme di Sant'Elena). They offer fresh fish, with a very good service. Good selection of wine for quality and at a very competive price. The restaurant is quite "simple" and in my opinion in really old-style ( it looks as it was in the 70s), but the quality of the food can worth a visit.
I hope of beeing of some help (and I apologize for my English) . I wish you a good stay in Tuscany!