It does depend on the plant and the location.
I don't bother loosening bedding plants, like Pansies, because they're not going to be around long enough for it to be a problem.
We don't get freezing weather so I leave the planting mix on the roots.
For trees and shrubs I cut any circling roots and at the bottom of the rootball to loosen.
For monocots, like grasses and palms, the roots don't grow fatter so there's no point in cutting them either.
