Impaction alone can be done without any decompensation phase in the vast majority of cases, but also in the vast majority of cases, it will lead to a slightly worsened bite.
So orthodontics aren't necessary per se, but desirable if your bite is important to you.
If I remember correctly, I was told that, for every 2mm that you impact the upper jaw, the upper jaw must come forward 1mm to maintain the existing bite, but obviously braces will usually be able compensate for this without having to move the upper jaw forward.