What aussie said is good advice.
The amount of benefit between level 5 to level 6 to level 7 is not much. I think the BBs were trying to make the high level trainers more of a luxury for the top teams with a lot of cash to spend. Most teams can't afford the bonus for a level 6 or 7 trainer, but with level 5, you can get some cheap (but with higher salary).
It's just a matter of how much $ you want to spend. Good luck.....