When accelerating away from a stop, I will usually shift between 3-4K. When just cruising at a constant speed I'm usually in whatever gear puts me at 2.5K depending on the speed I'm driving, if that makes sense. It is fun to run it through the gears when getting on the freeway, though. I think the highest you can go in each gear from the factory is 29MPH for first, 54MPH for second, and 80 in 3rd, if I remember correctly. Personally I have a 5 speed in mine, which keeps the shift points the same but just with an added overdrive. I never use 5th unless I'm going above 55.
There is nothing wrong with the way you have been driving it, so long as you're not dogging the motor and it does what you want it to. You probably get better mileage than me, if it's any consolation.
Side note: In relation to your other thread, I always try to keep my revs above 2K. Below that it really bogs the motor down.