How could the location of a radon abatement hole impact its effectiveness?
It's a simple matter of airflow, so the answer partly depends on the collection mechanism. 

If your system will rely on underslab media, such as washed rock or plain gravel soil, flow reduces exponentially with distance from the vacuum source. For this reason a centrally-located port would be most effective, by far. An alternative would be to have multiple peripherally-located ports. 

If your system will be a sealed drain tile type, it won't matter as much. Theoretically, vacuum will be roughly equivalent at all points in the pipe, and will therefore draw uniformly on the adjacent medium. In a large or complex tile layout some reduction in vacuum could be expected at remote points, but a true central port location is probably not an option as drain tile is typically installed at the perimeter of a foundation. 

Leaks in the system can dramatically reduce effectiveness on a localized or system-wide basis, of course.