Revell's 1/72-scale kit of the M1A2 provides idlers with different-length axles (long ones for the front, and short ones for the rear. As far as I am aware, the idlers should be equally spaced from the hull, allowing the track guide- horns to pass in front of the wheels. Is this correct?

If you mean the track return rollers near the top of the hull, no, they are staggered.

Scroll down the pics on this site, you can see the mounts for the rollers and how they are staggered. Cool site too

http://hotdigitalnews.com/before-the-battlefield-making-the-armys-abrams-tank/

Thanks ... of course, I meant the return rollers. I can't think why I referred to the idlers!!

Having noticed that the track guide horns pass across the outer face of the return rollers, I am wondering why the rollers are spaced differently from the side of the hull ... those at the front are further away than those towards the rear.

Okay, that is strange. I was going to say because of the additional armor mounted on the lower hull sides but the rollers are both mounted to this armor on the starboard side of the tank and they both still look staggered. Strange....

Another web page with cool construction pics

http://www.businessinsider.com/rebuilding-m1-tank-2013-11?op=1