I rode Sora shifters for 1500 miles and never had any issues. Sora is good stuff, so is Tiagra. I'm now using Ultegra, but prefer the thumb shifter on the Sora and think Campy uses the thumb, so that's probably where I'm headed next.

As for Trek putting Tiagra on the rear, I'm sure it's far more a marketing decision... allows sales people to say, "Yeah, even has the upgraded rear derail..."