This is why i don't check idle timing. I check at full advance, then just do a cursory check of idle.
Well, some exaggeration, sundie....
One of the purposes of the idle timing marks is to allow for a variation in the spread of the timing from idle to max advance. If the timing is less than the minimum mark at idle with it timed to the maximum mark at 3000 RPM, then it's time to replace the advance unit, something that for whatever reason, everybody resists doing.
Running the engine at some idle timing less than the minimum will result in sluggish engine performance at lower RPM's.
If the timing is not within the brackets for idle at idle then the only time it will be in correct time is at 3200 RPM and above. And, don't forget the possibility that the timing could be too far advanced at idle when it is correct at 3000 RPM if the springs are too tight, so it would be too far advanced at all speeds below 3000 RPM, which is also not a good thing. You can have some pretty destructive detonation at lower engine speed as well. In fact, detonation can occur just as you are starting out in first gear if it too far advanced at idle.
So, the timing has to be correct at idle as well as 3000 RPM. You shouldn't just take a cursory look at it at idle.
Additional notes:
Regarding the springs being too tight. If you check the timing at 3000 RPM and set it for the maximum indicated mark, you should also rev the engine up higher as well to ensure that the springs and the bob weights are at their maximum travel. That is the other purpose in checking for correct timing at idle and 3000 RPM, to ensure that the springs are not too tight, which would result in a too far advanced timing for all engine speeds below 3000, and the advance would continue to advance beyond 3000 RPM.
When installing new springs, you should check / set the timing first at idle, and then rev up to 3000 RPM to see if the timing advances. If you checked the timing at 3000 RPM first, and the springs were tight (all new springs are tight), then you would have extreme advance at all engine speeds below 3000 RPM.
Shine the timing light on the advance weights when setting max advance to see if the weights are actually at their maximum extension.