You are asking a question that has no one correct answer. In general, I would guess that your OnX maps are close. Specifically, each property is going to be different.
The federal surveyors surveyed much of the west 100+ years ago using very basic equipment. Theoretically, each section of land was supposed to be one square mile (5280' X 5280'). They put markers at each corner of the section and also 1/2 way along the section lines (called 1/4 corners). Smaller parcels of land are measured from and tied to these government placed corners. Their original measurements WERE imperfect. The markers they put at the corners were not always in the right spot. Here is the important part: THE SPOT WHERE THEY PUT THE MARKERS OF THE SECTION CORNERS AND THE 1/4 CORNERS ARE THE ACTUAL CORNERS, EVEN IF THEIR MEASUREMENTS WERE OFF. If it the new measurement between the section corners shows that they left that section 200' short, then that section is simply smaller than others. Even if you have a deed that says 640 acres, if a surveyor measures the land between the markers and tells you that you only actually have 625 acres, then that is the case. It works the same way if you happen to get 675 acres. This is true only for those original government markers, not necessarily for property pins erroneously placed by a land surveyor after the fact.
The point is that mapping programs probably have some good data that ties them roughly to the property lines/corners. Much of the west, especially on big ranches, has never actually been resurveyed since it was done by the federal surveyors 100 years ago. That means that much of the data shown on mapping programs is simply rough projections, not actually surveyed boundaries. Shooting an animal across a fence line because your mapping program says you can would be an unwise choice in my opinion.
Yes, some fences were built by ranchers, in their own favor. Some fences were built because it was easier to cross the ridge 50' to the south rather than over a cliff or rockslide. Most fences were built where they are because that is where the section corners told them to build it many years ago.
The bottom line is, if you really want to know where a particular property line actually is, you will likely have to hire a land surveyor who knows what they are doing to research and find it for you. I know that isn't the easy answer many of you are looking for.
Soup