Depends if we're talking about woody or non woody plants.
The 'trajectory' a root has made doesn't change over time. So a root that's grown in a circle will stay in that circle. Only new growth can go outwards from that point onwards.
For perennials this doesn't matter AS much since most will be dug up and divided regularly anyway to reinvigorate them. Any bad root growth will - with increased reinvigoration over time - be correct until the root system is all outwards.
For shrubs and trees this doesn't apply (or shouldn't). Any bad root growth will stay bad root growth and as secondary growth makes them thicker, they can eventually impede other roots or worse yet, girdle and kill the tree.
In both situations however you can slice the outer edges and bottom off before planting, but with woody perennials this doesn't guarantee a well spread out root system since you can't see what defects are present inside the root ball.
What I do with perennials: tip them out, turn them upside down, push both my thumbs through the middle (on the bottom) and tear outwards creating two 'flaps'. Spread out inside the hole, cover and water in.
