Side focus has the same function as objective focus, that is to focus out any parallax error.
Generally, the larger the magnification, the more parallax error can effect your aiming and accuracy.
First and foremost is what type of shooting you will be doing. Where quick shooting is required, such as some types of hunting, SF is impractical due to time constraints.
The general rule of thumb seems to be that for 10X and higher magnification SF can be beneficial.
If you are going above 10X and require a high level of accuracy, SF is the way to go.
A recent test I did shows that up to 200 yds and a 20X magnification SF is not a big issue. Going further out starts getting tricky without parallax error focused out.