Here are my observations, but bear in mind that this is largely a matter of individual preference. In my 2001 GTS, 55 is about as low as I would want to be going to be in 6th gear; anything less and you get a lot of driveline lash as you modulate the pedal (in the event you have to lift off the throttle due to traffic, etc). When I had my 97 GTS, 65 was the minimum cruising speed for 6th gear. I attribute the difference between the two cars to the diferences between creampuff and non-creampuff model years. Realistically, it's almost impossible to lug the Viper engine. Also, of course, these speeds would change with non-stock gears.
(These observations are for expressway cruising, no special hurry, point A to B driving. If I'm looking to do a more enthusiast-type driving, 6th gear (and even 5th) isn't used.