I could have sworn I used to do this in the past but the PTC people reassured me it's never been utilized before. When creating a sketch and you want something to be constrained symmetrically about a center line you currently have to dimension the line endpoints/center points then add the constraints after (dimension A to C; constrain A,B,C). What I'm proposing you do is automatically add the symmetrical constraint to an item when creating a dimension based off a center line (dimension A,B,C; symmetrical constraint is created automatically).
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