Heelcup adjustment is a little more complicated in that the term is often used to describe two separate and very different settings.
The first should really be called highback rotation (and often is) even though this adjustment is made at the heelcup. The idea here is that when an angled stance is chosen, the highback becomes out of parallel with the board's edge. The rotation adjustment allows for the highback to be brought back in sync with the edge.
The million dollar question is, should this be done? Does it help to have the back of the boot angled in one direction and the highback in another? There is no simple answer here. Highbacks are not always symmetrical, and boots come in a variety of sizes and shapes. That does not even begin to cover all of the different leg types that will enter the equation.
The very best thing to do is to experiment. Strap into your gear on a carpeted floor at the factory settings and feel how the highback meets your contact points. You will get a surprisingly accurate feel for this even with no snow involved. Now rotate the highback to parallel with the edges and retest. We have done this thousands of times for riders, and most have a strong and immediate preference to this test. Many times an "in-between" selection will be made and that is just fine.
There are no hard and fast rules to this, so let comfort guide you and don't hesitate to fine tune after a few snow sessions.
The second heelcup adjustment only applies to some bindings. These models have a heelcup which is free from the rest of the base. This allows the heel of the boot to be moved further out over the heelside edge.
This is a centering feature which should be used to attain the same amount of overhang for both the toe and heel of the boot. It also allows for different sized boots to match up better with the contours of the bindings (for example where the arch curve lies). If this adjustment is not available on your bindings, no need to worry, centering can be achieved by use of the disk slider and contour can be matched by a more specific binding fit.