I think it all depends on your plan and ability to compete. If you plan to train secondaries at some point, you have to decide when is the best time. For training purposes, the earlier the better because the later the longer it takes. As for league games, it depends. Say you want 2 seasons of secondary training, I think it would be better to have it early in their career because they're not good in league games anyway. If you wait with it after 2 seasons, you'll have a 15k center playing PG who could otherwise help your team. If you would have mainly secondaries with some ID every now and then, you'll have a servicable player (assuming IS is trained through 1v1 and some ID is mixed in), and you'll have trained secondaries faster, and kept salary lower.