Typically, in left to right languages, the most fixation occurs in the upper left portion of the screen. Users' attention then tends to form an 'F' pattern as they scan down the page.
Therefore, placing the core navigational elements in the upper left is often a pretty safe bet.
One point to consider here is that of widescreen monitors. Problems can occur if the core navigation is fixed to the far left or far right of the viewport, rather than being fixed to the side of a central content container (which often has a maximum width of no more than 1200 pixels).
On widescreen or 4K monitors this can become an issue. As the screen becomes wider, the navigation becomes increasingly disconnected from the content on the page. The user has to move their cursor further and further in order to move between the content and the navigation. This can often lead to greater inaccuracy and wasted interaction.
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For more detail, have a look at Fitts' Law which explains the mechanism behind this.
Stackexchange sites are a good example to follow. While the background of the top navigation 'wrapper' extends to the full width of the screen, the navigation links themselves are contained within a central container element with a max width of 1060px. Visually, this helps to reinforce the importance of the navigation while mitigating against the issues presented by Fitts' Law mentioned above.