I'm using the method of sewing together 3 strips of 2.5" across, then measuring, and hoping the end result is perfectly 6.5". For each foot / needle-position combination, I'll first press to the side, measure, then press open and re-measure.
Test 1: Zigzag foot at needle position 3.5.
Pressed to the Side: Just shy of 6 5/8
Pressed Open: 6 5/8
Test 2: Zigzag foot at needle position 3.3
Pressed to the Side: 6 7/16
Pressed Open: 6 1/2
Test 3: Quarter-inch foot (no adjustment)
Pressed to the Side: A few hairs more than 6 3/8
Pressed Open: A few hairs less than 6 1/2
The closest was using needle position 3.3 and pressing open. It seems when I first press to the side, it's not very accurate, but if I press open and *then* to the side again, it's much closer. That's a lot of work, but worth it if I want more accuracy and nesting seams as well.