Sway bar

[rpmerf]

Your suspension is a balance. Tightening up your front suspension increases under steer. Tightening up your rear suspension increases over steer. This need to be balanced to give maximum traction. Unless you are using custom parts, adjustable parts, or mixing and matching parts, it is unlikely most people will run into an unbalanced setup.

By adding a rear sway bar, you are tightening up the rear, which can make it more likely to over steer. I feel that as long as you keep it balanced and don't go too big on the rear, the risk is negligible, while ride quality will be improved.