A flush wouldnt be a good idea with that many miles at this point. I have seen numerous transmissions with higher miles get flushed and failed very soon after. You dont want to knock loose all that built up crap in there. I would say a trans cooler and regular flushes, not just fluid drain and fill, could improve longevity slightly if done from when the car is new. But at 170k, I dont think your going to see much, if any benifit, unless of course you are towing something.
Are you having any issues that are causing you to think your going to have problems soon?
I would bet that the ones having problems havent followed fords recommend maintenance schedule for their transmission. Or they dog it, or worse, the ones ive seen that leave it in nutreul and then slam it into drive to do a burnout.....sad. Preventive maintenance and common sense will always save you a whole lot of time, money, and in some cases tears and bouts of rage!
i have one on mine, was needed after the stall since they generate a lot of heat, but even from the factory they can make too much heat if you abuse it enough, it's simple to install really, just attached the new hose to ends of old lines with ran around the drivers side of radiator to the front where it attached to the cooler, i just mounted the cooler to the ac condenser with the supplied ties and plastic "washers" i guess you could call them...
i did mine as a complete bypass of the factory system to keep it away from the hot radiator