YES. Definitely.
A+ with 4 potential balls beats A- with 5 potential balls.
If plan on having more than 3 trainees then it's a no-brainer, since you will never reach his cap.
I've been training 2 positions since I started this game, now my players are well rounded and I'm beating teams who pay 3x more than me for their starting 5's salary.
I have a couple of MVP's who could just as well be perennial-allstars, it wouldn't make a difference to me because I'm not selling them.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme