Yeah, that throttle plate should be pretty much closed all the way. There is a bleed bypass hole that exits behind the throttle plate - that will allow enough air through to idle purposes. I agree, not worth taking the throttle body apart, unless you know that is the problem. The spring might not be the part to worry about, more likely I\'d worry about damaging the sensitive TPS sensor.
Throttle bodies, in general, do not go bad. Mechanically, they are pretty simple. Usually it is the electronics, like the TPS sensor, that could be offset or reading incorrectly, causing the car to mis-read how far the plate is open. That would definitely cause idle issues as well as driveability issues. From your description, could be as simple as the idle stop screw was messed with by accident.
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)