How taut did you have the backing when you pin-basted the sandwich? Reason for asking: If it isn't taut, then the grid can end up puckered in one direction and you won't notice until you have one side quilted with the Stitch-in-the-ditch method. If it is nice and taut, then I would roll half of it and starting with the center line, SID the straight lines.
Could you do a serpentine line that meanders around the ditch? It may be a better fit to go with the FMQ design?