Jshep ~ did you pin or spray baste, your sandwich? If you pinned, are the pins approx 3-4" apart. Your fist can be your measure-er (mine are on the larger size). If to far apart, you can increase the odds of puckers. Spray basting, make sure to smooth both sides of the quilt.
To stitch in the ditch, is not really 'in the ditch' it is right next to the 'ditch', 1/8" down to 1/32" or as close as possible. Back when you pieced, and pressed to one side, you can feel the 'bump' of the seam. SID is done on the 'lower' side and not on the 'bump' side. I sure hope that is as clear as mud ;)
IMHO when I SID it takes more work, then when I do FMQ. BUT the FMQ took a while to find "that sweet spot" that I was comfortable with.