Here is the paradox with training:
You want a high level trainer when you are training younger talent because it is more effective. As the players get closer to capping it does not make sense to overvalue a trainer.
However, when you have a young group of talent to train, you usually do not have the cash flow to buy a superior or higher trainer at a decent price.
So, if you want a level 5 and up trainer you normally fall into a specific group optimally: that is you are a rich team looking to make the most out of either someone you drafted while tanking or an MVP+ 18 year old you purchased.
My philosophy is this: Train the guys until they get close to capping, then cut off training until you need to make a title run. During the title run, push their main skills up to get an effective economic bonus over your competition.
That strategy does not require a level 5 trainer. Level 4 is enough to produce close to capping but not capping players in the range of Superstar or less potential. IMO is enough to ride on except in Div 1. There you may need an MVP or greater to lead the team. But then again maybe not if you want to run a salary efficient club.