Typically you add the border to the long sides first, then the short ends. This is because it takes less length of fabric (yardage) -- assuming you are cutting on the straight-of-grain, parallel to the selvedge. Straight-of-grain is a more stable cut than cross-grain (selvedge to selvedge) which is why it is preferred for border strips.
If you add borders to the short ends first, this makes the long ends even longer. If you don't have enough yardage, then you may have to piece your border strips.
In other words, it reduces the yardage you need to purchase.