I always use midfield if i want to get a general feel for the car and create a general setup. Dispite all the folks saying nurburgring:
a) its way too long to get consistent lap times
b) i found that it has many more high speed sections than most other tracks in the game, and therefore requires a longer gear ratio setting than the average track
c) it wasn't in any of the previous GT games and I personally don't want to learn an entirely new (4 mile?!) track simply to test my car.
I would suggest that in order to get the best feel for the changes you are making, use the track that you know the best, the track you can make the best times on. It has always been midfield for me. you definately don't want to test on a track that you suck at =]
also if you are having trouble with a certain competition it is best (obviously) to tune on the course(s) that you will face there for the best specific setup.
-Shmak