It depends on league level and personal preference. I don't mind training a decent player that is at least star potential, but I wouldn't make them my primary trainee. Some people, especially users in higher divisions, are a lot more picky and only train high potential players (MVP, HOF and to a smaller extent because of its rarity, ATG).
If you want to make some quick money, then you might not mind training star and allstar potential players. If you are training long-term, you probably want to start with PA/SS potential.