Well, there should be more negative than positive, because older players should pretty much all decline, whereas young players should be about 50/50. Plus injured pitchers have a heightened chance of decline during their injury (which is realistic)
(On that subject, one of my recent draft picks is a SP who got a season-ending injury after the first couple sims. Since then, he had avoided any talent decrease, as if the game didn't notice him. Then I'm looking at my minors report and it says that he should be ready for AAA. Ha, I think, doing rehab after shoulder surgery got him ready for a higher level. For fun, I go ahead and move him up to AAA last sim. It seems like the computer noticed the transaction and though "wait, he's injured! I had forgotten about him!" and immediately gave him a talent decline. I should never have done something that brough him to the attention of the gods of talent modifications, who are vengeful gods.)
Anyway, I've generally found the talent changes for young players to be pretty fair. I can attribute a lot of my recent success to talent increases for players like Roberts, McDill, Guerrero, and Nissen. And the talent increase to Norberto Cruz made him a good chip in my trade for Arispe. Of course, I've lost a lot of prospects to talent decreases, but, like I said, it seems fairly even to me, good vs. bad on talent changes to young players.
As far as changing the setting for talent changes throughout the league, I'd say it's unnecessary, but whatever - the setting is the same for all of us.