With my Nolting Longarm you can clamp it down (Lock it) so it only does a straight line left to right, or 'lock' it the other directions to do a line top to bottom. With rulers you can do the diagonals.
A lot of times with SITD your quilted (pieced) seam is not exactly straight anyway so it would be a matter of guiding the longarm along your seams.
I also agree with the post of if you are longarming a quilt you want more than just SITD.
PM me if you have any questions.
Phyllis
QuiltingGrannie
Quilter's Pantry