It depends. Some guys obviously need it, some don't.
If a QB is truly elite -- P. Manning, Elway, Marino, Bledsoe, Roethlisberger, Ryan -- from the time he enters the NFL, no grooming is needed. Roethlisberger is the only guy on this list who had a great team around him in his first year. But all these guys pretty much started from day one. Ryan's "eliteness" is still to be determined, but if 2008 is any indication, he will be that guy.
However, there aren't many elite QBs these days. Guys like Palmer (Kitna), McNabb (Pederson), Rivers (Brees), Cutler (Plummer), Rodgers (Favre) and so-on needed to be groomed/given a chance. They are good QBs, but probably not "elite." The last guy to be groomed was Rodgers, but only because he was behind Favre. Palmer was the last guy to truly be groomed. He didn't attempt a pass in 2003.
I assume the QB you're debating is Mark Sanchez. With such a limited college resume, I don't think it would hurt for him to sit for a year or two to learn the position and nuances of the NFL. However, he could start from day one and shock the hell out of us. My gut feeling is that he sits for at least a season, ala Carson Palmer (just a coincidence they're both USC QBs).
Some guys are better for it. Some guys don't need it. Some guys just don't make it, whether they are groomed or start from day one.