I think it's kind of difficult for a Tiptronic owner to drive the car in quite the same manner as one with a 5 speed manual when just cruising. It doesn't help the car to be in 5th gear at 60, but not too many people buy an automatic because they want to keep it in 4th themselves, so I'm sure plenty of Tiptronic-equipped cars get "babied" in this way simply because that's the nature of an automatic.
My mileage is on the lower end, the last three tanks have been a pretty consistent 20.5 mpg, and it's a '97 with the 2.5 and a 5 speed manual, but I also put the car through its paces when I go for a drive, so I know that has to do with it to a point.
75mph in 5th puts me right around 3000 rpms, and I rarely let the car stay below 3000rpms for a prolonged period of time unless I don't have a choice or don't want to attract the wrong kind of attention because I'm in 2nd gear at 4000rpms doing the speed limit in the city...or 4000rpms in 5th on the highway (~100mph)
Folks have said having their engine computer tuned made a significant difference in their fuel mileage for the better. That's something to consider doing even if you don't do other modifications to the car, as those extra miles per gallon will add up over time.