Sferics and tweeks can be heard almost anywhere, but whistlers and VLF emissions are more prevalent in northern and southern latitudes. These signals are rarely heard in the tropics. Since whistlers and VLF emissions propagate along the earth' field lines, they tend to exit the ionosphere at more northerly latitudes and thus are more often heard there. Whistlers tend to be most prevalent around a geomagnetic latitude of 50 degrees.

In the USA there seems to be better Natural Radio reception in the Western States as opposed to the eastern states and midwest. I don't know of any studies that have been done on this, but it may be due to the fact that there are more open spaces away from power lines and thus better listening locations in the west. It also may be due to some other phenomenon.