It is more comfortable to look downward than look upward, while the head is in a fixed position. If you often need to look upward for longer than a glance, you are likely to tilt your head back a bit to relieve strain on eye muscles (trading it for strain on neck muscles).
Often people compensate by resting their head on a headrest and leaning back against a back rest. Often this causes shoulder and back strain. Then they use an armrest or palmrest to type, causing RSI from static loads on bent wrists.
There are two main ways to avoid static muscle loads that cause discomfort and long-term problems. One is to regularly change position. Another is to default to a neutral and comfortable basic position.
Do you know what the relative trade off is between eye muscle strain and neck muscle strain? Although intuitively the head is a lot heavier than the eyeballs the neck muscles should be much stronger to compensate. Perhaps the neck spinal column will be deformed after a while?
You can reduce both by lowering the display so that its top is roughly at eye level (or maybe up to 5° above or something), and tilting it backward a bit.
Ideally you want to find a neutral position where there is as little static load on any of your muscles as possible (including the eye muscles used for accommodation (focusing), convergence, and turning your eyes around). Temporary dynamic muscle loads are fine, but static loads get tiring and lead to long-term problems.
OK. I didn't write that with much precision. What I mean is assuming you are sitting comfortably upright in a neutral position and your gaze is level, horizontal and perpendicular to the floor, tilting your head back so that your line of sight is at a 5-30 degree upward angle is a recipe for eye and neck strain because your eyes have to work harder to relax and refresh. It is much more comfortable and natural for your line of sight to be slightly below level, several degrees or more angled downward.
So what this means for most people is: don't prop your monitor up on a stack of books or an additional stand, simply place the monitor on your desk and adjust the attached stand so that the top edge of the monitor is level with your natural, relaxed line of sight and then tilt the bottom of the monitor up very slightly as described by someone else above.
Can you clarify what that means as in the ideal case the head is not tilted forward to begin with.