You can define curves in Style to be symmetrical across a selected datum plane.

User Interface Location: Click Model > Style and then select Curve or Curve Edit.

Release: Creo Parametric 5.0.0.0

You can also watch this video on the PTC Learning Connector: Creating Symmetrical Curves in Style

In Style, you can easily create or edit an existing curve to be symmetrical across a selected datum plane by selecting the Symmetrical Curve check box on the References tab. The symmetrical side of the curve that is created appears as a dashed line to distinguish between the original side of the curve. When modifying the definition of the curve, you can only edit the original side, however, the curve updates dynamically across the mirror plane.

You can remove the curve symmetry and choose if you want to keep both sides of the curve across the mirror plane or the original side. If you remove symmetry and keep both sides you can edit the curve without influencing the other side. As a result, you can create a clean single curve across a mirror plane which is helpful in creating surfaces with good quality.