1 sensor in the middle will actually always read the lowest temp of all the chips MINUS the delta T between junction and Heat sink (could be many degrees C). The BEST thermal monitoring uses one sensor per chip, and always makes critical decisions (like over temp shut-down) based upon the highest reading - not the average. Just because the average temperature is 'safe' doesn't mean that one hot chip isn't about to explode due to an over-temp condition.
Enigma
I am considering using a heat sink that covers two rails, one on each side of the board and is open in the middle 1-1/4". A sensor in the middle won't work for that. We could use an analog switch and place a sensor at each quadrant of 4 chips.