Poor tracking is going to be more obvious in images with smaller image scales, so in that sense I think it is true that long focal lengths require better tracking. I agree that this doesn't imply that you need to guide at a longer focal length. Here's an article that claims that a very short focal length guide scope is sufficient unless you have outstanding seeing:
http://www.wilmslowastro.com/tips/autoguiding.htm
The other aspect to consider is that there are other causes of bad tracking like differential flexure between a scope and guidescope or movement of a primary mirror. A common solution to these problems is off-axis guiding. The benefit from OAG is not that a longer focal length is being used - it is that the guider is seeing exactly the same field as the imaging camera, subject to the same mechanical variations.
Cheers,
Rick.