Maps will automatically try to avoid as many toll roads as possible, as long as it can find alternate free routes to the otherwise tolled sections to still get you to your destination within a reasonable amount of time. However, the definition of "reasonable" seems to vary among several conditions, including the distance of your trip. For example, a 20 minute delay to avoid tolls on a 150 mile trip is considered reasonable, but for a shoter trip of say only about 40 miles, that same 20 minute delay might not be considered reasonable and it may suggest just eating the toll.
Alternatively, Maps does provide the top 3 routing options it finds when you give it a destination. You can select any of the 3, but if you go off course and Maps has to recalculate, it will automatically resume routing with the top route it finds.