Almost every day now, rookie safety Eric Reid is making a positive impression on 49ers coaches and teammates.His high football IQ and his ability to learn quickly make it apparent that even though this is Reid's first NFL training camp – and he's listed at No. 2 on the depth chart at free safety behind C.J. Spillman – he could be the opening-day starter on Sept. 8 against the Green Bay Packers.But as the 49ers' first exhibition game Thursday night against the Denver Broncos approaches, veteran strong safety Donte Whitner has said the best thing for his young teammate's development would be to play as many snaps as possible against front-line players in the exhibition games – so he can test himself and even fail.Whitner told reporters the summer games can give Reid some valuable, cost-free lessons, and the first could be against the Broncos' Peyton Manning, one of the NFL's best.Making mistakes against Manning would be valuable teaching tools."I think it would be great," Whitner told the Bay Area News Group. "The only thing it might cost you is a little confidence if you get beat. But it won't cost you in the win column, and this is where he's going to gain his...