Really, the world won't implode if your lines cross. It's just an aesthetic preference of some people (and possibly would matter if you entered your quilt in a competition). Some free motion designs have crossing lines, some don't. In stipple or meander quilting (I would call it meander, but maybe I'm not up on the technical terms) I think the general guideline is not to cross, but if one or two or ten little spots have the lines crossing, I seriously doubt anyone will notice, and it will definitely not affect the sturdiness or usefulness of the quilt.