I've heard some different opinions on this one, but I can see where the synthetic oil does a better job at reducing the friction needed to seat the rings. BUT, If the bore is round and has the proper finish, and the ring package is of the right material, tension, and has the prober ring land clearance, then it should take very little time for the rings to seat. If the factory feels confident enough to run Mobil1 in the Vette, then I can't see it being a problem with any LS engine.
My feeling is that waiting a few thousand miles is probably a safe thing to do. Change the oil at 500 miles or so, then again at 2500-3000. At that point the change to synthetic should present no problems.
I think that the factory QC has greatly improved over the years, and the rings are probably seated before the car ever hits the transport, so this probably isn't even an issue. Better to be safe than to be a "huffer".