One should keep at least 1 area at 3 watchstones, 1 at 2 and one at 1 if you want to avoid wasting higher tier map drops.
If there is for example no t12 visible, the next lower visible tier will drop. If there is only one 4socket region and everything else it without sockets, it will drop as the highest available tier without sockets (should be around 5? didn't check).
There might be some overlapping (4socket and 3 socketed areas can both have t14 or similar).
A fully socketed atlas for example can only drop down to t13/14? (not sure about lowest 4 socket tier) and t1 (all inbetween get demoted to t1 since no other tier is visble and the t1 maps can always drop).