Here is a pic of a '87 170's stringers.
I don't think they redesigned the stringers much after they put the fuel tank below the sole. You can see that there are 2 stringers with a 30+ inch gap between them. More forward you should have a bulkhead fore and aft of the tank and this supports the sole in those areas.
What you may think is soft decks could be the unsupported areas of the deck between the stringers. There is usually an open area around the beginning of the casting deck also. The fuel tank "lid" is also unsupported and is alway giving a spongy feeling.