If the clutch bell isn't shimmed properly (too tight), it could cause drag that would make the car move at idle.
If the clutch bell bearings are dry/dead/worn, it could cause drag that would make the car move at idle.
If the springs on the clutch shoes are old/overheated/weak, the shoes will swing out too low in the RPM's and cause it to move at idle.
If the shoes are worn or hanging up on the flywheel, they may not be returning to their resting position fully and cause it to move at idle.
If the idle is too high, it will cause it to fling out the shoes and it will move.
If the shoes are on forwards or backwards, shouldn't really have any effect on idle. Supposedly, backwards shoes cause a harder engagement. Not sure how true that is, but it will deform the shoes. I tried that many years ago with a t-maxx with the traxxas 2 shoe clutch.
You could post a photo or two of your clutch bell and the shoes with the bell off, then we can help more.