"Inevitably, places near Piazza San Marco and the Rialto can have some lower standards."
maybe rule of thumb, but there are a number of quite good reasonably priced restaurants within a few blocks of those sites. also, with the exception of the ever shrinking blue collar neighborhoods, almost everyplace in Venice is a tourist site.
in my many visits to venice as an obvious tourist, i have only received 1 overbilling. was a beauty. wife and i had a 150000£ lunch, and were billed 1500000£. do not know what would have happened if not caught at the restaurant.
in my experience, each community has a different "style" of cooking and different foodstuffs. much is due to the demand for "fresh" foodstuffs.
a tuscan restaurant in venice is considered "foreign", as is the
Adriatic restaurant in bologna.
the restaurant offerings in Parma, Modena, bologna, and ferrara are quite different, and they are all in a 50km radius. some are inexplicably strange. Gattinara(about 50miles NW of Milan) serves piaia.