Well Maravich, I didn't get a level 5 trainer until I had the right group to train. My theory is that a higher level trainer can get better results out of 20-22 year olds. Again, a theory.
I would look at the average for a level four trainer and once you start to go well above average, I would make a change. That is what I plan on doing with my level 5 guy, but I got him on the cheap and I'm going to ride him out as long as possible.
I had a level 10 for a while in the old system, so far in the new system I have had two different fives. I plan on two different sixes next. I say two because you'll have more numbers to compare and it takes away certain variables.
I would also theorize more trainers will be hired after the draft, so if you're going to change, do it now.