It's hard to think in those terms and give you a correct answer because there really isn't one. A player with what looks like high/decent skills in division 3 will suddenly look like a middling player if you promote to division 2. The higher you promote, the better your players need to be. But at the same time you obviously need to fit players you can afford onto your roster.
But if you're training SF's, try and keep them balanced. That will keep their salaries affordable. If you train one skill several levels above the rest then you're just paying a higher salary for a player that likely isn't worth it.