Every time I've started a SA, I've wound up thinking it was wasted training. I have one I'll return to SA training next season when I expect his progress to slow. Hopefully he'll maintain good form over 2-3 more seasons, giving enough time to finish a SA and make him more appealing in the transfer market.

Believing that progress slows with each quality star gained, you might train until just before the next star (99%, for example), then switch to SA training to maximize progress.