Hi Bridgitte,
1. for the head 6 subdivisions will be enough, for the cube since it starts off with four faces vs the few thousand on the level 0 of the head
you need at least 8.
2. If geometry gets awkwardly spaced (long rectangle , or any shape not close to a quad)
while sculpting lower levels and using some brushes like bulge at large strength etc the higher levels at that point will not have enough resolution for the knife tool as that area is essentially low res compared to other areas.
Solution smooth your lower levels of the stretching polys and get them as close to geometry around them as in quad shaped and the artifacts will dissappear.
3. using heavy strength on lowres with some brushes will cause artifacting on highres , use a strength thats just right for the particular level, lower levels lower strength ( this is esp true of knife tool) . look at how the sharp lines in level 8 have softened in level 6 , if i dig into level six , the lines will not look clean in level 8, leave sharp stuff for highest levels.
in settings put the steady stroke on , i use the value 1, it makes the path smoother.
make sure you have wireframe turned on when troubleshooting polyspacing ( right click outside model in viewport and toggle wireframe on)
img 1 how it should be at level 8
img 2 artifact caused at level 6 will not be strong at level 8 but it wont be very good either.
img 3 at lower level 3 the poly is too far stretched compared to surrounding polys leading to the artifact on higher levels.