I've seen this topic discussed many times over in my 7 years ownership of the V6. I've also had my fair share of cooling issues which resulted in many hours of diagnostics and replacing the thermostat (a couple of times), radiator and water pump!
In my experience...
Car runs at 70(ish) when in free flowing traffic at over 40mph and at just over 90 when stationary. The fans then kick in and bring the temp back down to just below 90. This cycle from above 90 to below 90 takes about 3 minutes. It has operated like this from when I took ownership of the car at about 7 months old.
When the thermostat fails, the car takes an age to warm up, even to 70. When running in free flowing the temp will drop below 70, even, as I experienced in the depth of wither to below 50 which caused the engine management to give an error code. On replacing the thermostat the temperature charactristics returned to as I described above.
I'm confident to say that:
~70 at pace and ~90 in traffic = Normal
<70 at pace and much >90 in traffic problem to be investigated.
There's a hell of a lot of paranoia on this topic and I'm sure a lot of people have brought new thermostats unnecessarily due to this paranoia!!!!
Red 159SW 1750 TBI TI