If I had my choice, I'd run them parallel as it's 'easier' to set both sides up identical. Ackerman can be introduced with an off-set servo horn if there are issues with tension.

I think my 2 pull-pull planes are setup with the lines crossed - they're ARFs, and the slots were not in the right place to run the lines parallel.

Crossing the lines works, but I don't like the non-symmetry.